Am i doing something wrong my son just seems restless!?

Okay well I just seem to have such a hard time getting my son in bed for the night he just fights me the whole time I try to burse him rock him and make him as comfortable as I can but everytime he's almost asleep he jerks up or pulls his passy out it has been taking me about 2 hours to put him to sleep!! He's 5 months old and never seemed to have this problem before until recently and its driving me crazy! I know he's tired he just won't sleep! Please any suggestions on how to make him calm and easier to get to sleep