Have a 2005 impala and I'm having problems with the headlights. When i turn them on they don't always turn on. They will work sometimes but they don't work sometimes

Could be any number of reasons why this happens . The headlamp switch , the dimmer switch the headlamp relay , the BCM - body control module , wiring , connections , ground etc..... Without testing no one on here could say for sure what's wrong . Do you know how to test electrical circuits ? Viewing wiring diagrams then using a DVOM - digital volt ohmmeter to test the headlamp electrical circuits . Possibly hooking up a scan tool an checking headlamp switch input data to the BCM an seeing if the BCM is energizing the headlamp relay .
Free wiring diagrams here http://www.bbbind.com/free-tsb/ Enter vehicle info. year , make , model an engine size. Under system click on lighting , then under subsystem click on headlamps . Click the search button then click on the blue links .

https://www.youtube.com/watch?v=SO-eKuhCOqc&t=125s

https://www.youtube.com/watch?v=YaYtCVBodpw&t=98s

Headlamps
You can use the headlamps two different ways.
• Place the headlamp switch in the HEAD position for normal operations.
• Place the headlamp Switch in the OFF position for automatic lamp control (ALC) operation. During ALC the low beam headlamps are on for daytime running lamp (DRL) operation in daylight conditions or low beam headlamps and exterior park lamps on in low light conditions.
The DRL/EXT Fuse in the top underhood junction block supplies battery positive voltage at all time to the coil side of DRL relay, left low and high beam headlamps. The EXT LTS Fuse in the top underhood junction block supplies battery positive voltage at all time to the right low and high beam headlamps.
For normal manual low beam operation when the headlamp switch is placed in the HEAD position, ground is applied from G203, through the low beam contacts in the headlamp switch to the body control module (BCM).
The BCM then applies a ground to the headlamp relay control circuit. This energizes the headlamp relay in the left I/P junction block allowing ground from G201 to be applied through the switched contacts of the headlamp relay, through low beam contacts in the headlamp dimmer switch then to the left and right low beam headlamps, illuminating them.
For high beam operation the headlamp dimmer switch is placed in the HIGH position, ground is then applied from G201 through the switched contacts of the headlamp relay, through high contacts of the headlamp dimmer switch to the left and right high beam headlamps, illuminating them.
When the dimmer switch is placed in the HIGH position, the indicator lamp input to the BCM is pulled low. The BCM then sends a class 2 message to the instrument cluster in order to illuminate the high beam indicator lamp.
If the headlamp switch is left in the HEAD position, the inadvertent power control feature will turn off the headlamps after 10 minutes after you turn the ignition switch to the OFF position. If you place the headlamp switch in the head position after the ignition switch has been turned OFF, or if the ignition switch is in ACCY position, the headlamps will remain on until you turn them off or until the battery runs dead.