Recently, it's been taking longer to get the pilot to light. Now, after we light the pilot light, the heater won't turn on. When I turn the dial nothing happens. There's just a pilot light. Any suggestions?

does it turn without being pushed in?

If it's not electronic. Turn the dial to pilot and push in the pilot button and hold it, then put a flame to the end of the tube under the water heater, continue to hold until the thermocoupler warms up and release. If it goes out do it again only hold the button down longer. The turn the dial back to on..

If the dial is a rotory type of gas valve it should move to the pilot position, light the pilot and then continue to full. If the dial is stuck and will not turn at all it can sometimes be loosen by spraying some oil into the spindle. This would require several sprays and try very gently to turn but do not add excessive force - that could break the spindle. If it turns to pilot but pilot does not stay on this is normally a worn thermocouple, simple and cheap to replace. Found this site very useful, patio heater problem solving.

1) Get more air to the water heater and see if problem continues.
2) Read the following pdf-s for things that might apply to your situation.
http://waterheatertimer.org/images/Negative-pressure.jpg
http://waterheatertimer.org/pdf/Attic-Ventilation-and-Pilot-Outage-in-Gas%20Water-Heaters.pdf
http://waterheatertimer.org/pdf/Pilot-light-will-not-stay-lit.pdf

3) Read pdfs above first, and then Add a comment with missing information below, for possible do-it-yourself help.
Does this water heater have a blower?
Is gas valve electronic? Or does gas valve have a dial?
If gas valve is electronic, have you checked outlet with circuit analyzer?
Did pilot light work correctly for a while? If so how long?
Where is water heater located? Hot attic? Closet? Garage? Is area dirty and dusty? Has air intake been cleaned?
What color is pilot flame? Clear blue or yellow-or-orange?
Have you inspected inside of combustion chamber for dripping water?

If it is this heater, then you must:

1- turn the toggle switch to the ON position.
2- Turn the gas valve dial to the PILOT position.
3- Depress the pilot button while pressing the ignitor until you see a flame. Continue to depress the pilot dial for about 30-60 seconds.
4- let go of the button, and the pilot flame should stay lit. If it does , then turn the gas valve dial to the ON position.
5- Set the temperature and your done.

First turn the dial on the hot water heater to the off position and wait for 5 minutes. Then turn the dial on the water heater to the pilot position. Press in the dial and use a gas grill lighter, long wooden match, or similar device to light the pilot. Once the pilot is lit, continue to hold in the dial for about 1 minute then release and turn the dial to the temperature setting that's comfortable for you.

Sometimes when you turn a gas heater off for some time, if the source tank has been turned off, the line will bleed the propane and the line will contain mostly air. This is especially true if the supply line is long. In order to light the pilot, you will have to hold the pilot lighting button down for a while (a full minute or longer, depending on the size and length of the supply line) before pure propane arrives at the pilot light jet. You should keep a lighted lighter at the jet so that when the gas arrives, the lighter will light it. After the pilot lights, you will have to keep the button pressed long enough for the thermocouple to get hot enough to set the pilot to stay on. When it is hot enough, you should be able to release the pressed lighting button and the pilot should stay on. If it does, you should be able to light the heater. If the pilot jet never lights, gas is not getting to the regulator. If the pilot will not stay lit after a period of time waiting for it to heat the thermocouple, the thermocouple or the regulator may be bad and need replacement. :-)

The question is? I'll assume you cylinder is the water heater, right?
On the heater (water) you will find or see the red controls and a sheet metal cover over a hole. Remove the cover and this is where you light the pilot light. Press down on the red dial and turn it to pilot. In that position you will be able to press the red button . Geyt a long match or bbq lighter, then, holding down the red button dow for a minute or so and place a match or lighter into yhe area where the lines go The flame will catch the gas and lite the pilot if yo simply move your glame around. When the flame catch on the pilot, continue holding the button down for sveral minutes until the buld the flame isheating gets hot. Then release the buton and turn the main dial to on. The main burner should fire up. If not, and the pilot goes out, repeat the process. The bulb in the pilot flame is a thermocoupler that shuts all gas off if it doesn't detect a pilot flame, so hold the button down longer. If the pilot goes out when you release the button, The thermocoupler is no good and needs replacing. Good Luck_Ned_

You need to bled the line longer and when you get lited hold down for a good minute.When the gas gave out ,there could trash in the line.I would take the line off and blow gas through it undtil you can smell it.

you should have to push the dial or a seperate button in to manually open the pilot gas valve. If you find this not to be true and you have a spark either the pilot burner assembly needs to be pulled and cleaned.