I have trouble sleeping sometimes after workouts and just in general for various reasons.
Sometimes I let a source of stress bother me to the point where I lose sleep (issues with my son's mom, for example).
Sometimes I've just been going to bed too late for a few days straight, so my body gets used to it and it becomes difficult to get to sleep earlier.
Sometimes I find it difficult to get comfortable. I'm a side sleeper, if I have a sore shoulder or something, it can really affect my ability to get comfortable while lying in bed.
I've read and suppose that personal experience supports that drinking before bed can keep you awake. Like if you have like a glass of wine, you should be fine, but if you're drunk it could throw off your hormones and keep you up (not to mention making you have to pee a lot).
The pre-workout supp that you take could keep you up, too. If you take your pre-workout too late in the day, it can affect your ability to sleep hours later. This has happened to me and is the most likely reason I have a hard time sleeping on workout nights, sometimes.
Being amped from the workout can also have a similar affect. I play soccer on Sundays. There have been times that I played a late game and came home and couldn't sleep. I'm guessing the adrenaline rush from running hard for an hour wasn't over, yet.
There are a lot of things that can affect it, so you may have to look at every possibility to figure out what the problem is. There are some articles on the site that talk about sleep quality that you may want to check out (if you haven't already). Those could answer some questions. Just go to the search feature and type in sleep and you should find some (older threads that may also have some answers will also pop up).