Can anyone help me figure out what wrong with my mercedes?

My 2007 mercedes benz c280 4matic has difficulty
starting at times. It will crank over, but won't start. My bf
will dump a cap full of gas in the carborator or spray some
starting fluid and after a few turns of the ignition it starts
up sometimes. We have replaced the sensors in the front
of the motor (can't remember if those are cam shaft or
crank shaft position sensors but I believe we have done
both. Anyone have any clue what else is wrong with the
car? What else needs to be fixed next?