2008 frontier Nissan sometimes will crank but will not start does not do all the time new starter but still having problem

when was engine last serviced ?

If your saying it cranks good but won't start, have a helper crank it, while you check for spark at the plugs. If spark everywhere, check proper fuel pressure and fuel injector pulse.
Now, if your saying the starter motor won't crank the engine? Do you have a security system on this vehicle?
I looked info for v6, your vehicle, there is a park/neutral relay, automatic tranny. Usually with testing, there is a way to bypass security system, where applicable, to see if starter motor will crank the engine. Even you get it to crank, that doesn't mean it will start? If you try that, make sure tranny is in park or neutral and parking brake is set.

You may have a failing starter. After about 5 - 7 years the battery begins to weaken it will allow your starter to kick but the starter itself may be too worn and too weak to keep your flywheel turning.

System is most likely full of air. Sometimes continuing to crank the engine will purge the air and it will start but this can be hard on the starter motor. You may have to remove the fuel filter and fill it with fuel to prime the system.

Slow cranking will make it hard to start this is not anything to do with the distributor but either a lazy starter motor or a failing battery ,put a drop tester on the battery and see if its ok if so then use a heavy duty induction amp meter on the earth lead and see what the starter motor is drawing ,if more than 180 amps then starter is the problem

First thing to check would be the starter solenoid-mounted on the starter. In the starting circuit, that is the most high use/high wear item. A tech would wait for the no start condition, then check the solenoid wire for power when key is held in the cranking position. If power is there, it indicates a faulty solenoid. If no power, trace the signal back to the ignition switch-from solenoid to starter relay to clutch switch to ignition switch.
There are contact points (plates, really) inside the solenoid that become pitted and burned from use, and eventually wear down or out.

my truck won't start but .. no click just a little kinda grind and then stop .. is it the starter ?

Make sure you have 12 volts from the positive side of the battery to the starter. Check the small wire on the starter solenoid and make sure you have 12 volts when cranking engine. If voltage is correct, the starter is bad

Does the starter crank the engine over? Saying your truck won't start could mean that the starter spins be the engine won't fire. If the starter is not spinning is it at least clicking?

I had the same problem. Turned out to be the clutch interlock switch mounted under the clutch pedal. A combination of the clutch pedal and switch mount being there for 15 years and had bent just a little not allowing the switch to be fully depressed and my floormats getting behind the clutch pedal not allowing the pedal to push the switch all the way. You can press the switch by hand and turn the key (making sure the truck is in neutral of course). If it starts reliably every time you found the problem. I adjusted the switch out by disconnecting the wire to the switch and loosening the 14mm nut behind the switch and turning the switch out a couple of turns. When moved out a bit and keeping the floormat from behind the clutch pedal, voila, starts every time. This fix came after a new battery, new starter, new battery cables and still the intermittent no crank was present. Start with the cheap/free stuff first and move on from there if needed :-)