Lucid Dreaming question

For the past few weeks i have been working on my dream recall and have gotten fairly good at it remembering 2-3 dreams a night. However last nite i took a melatonin (5mg) just to help me fall asleep earlier, and then also a b-12 vitamin. During my dream it was just me and all the people i use to know in a classroom (pretty boring actually) then i wake up in my bed and spot a cockroach-looking bug on my bedpost i swatted at it because it surprised me but the way it moved actually made me laugh because i knew it wasnt real. so i take a look at my hands and ask myself, well actually told myself i was dreaming and sure enough my hands were almost transparent. So i was like "hell yeah" lets get out of this bed and fly or have mad sex or something. At this point i knew i had a false awakening, but when i tried to get out of my bed my movement was very very limited the more i tried to get out of my bed the harder it was i was entangled in sheets basically until it was a crawl out of bed, i made it a few steps then gave up and basically made myself wake up. I recorded this dream in my journal and then listened to some trance music to put myself back to sleep. It was around 6-7am, When i fell asleep again and had my second dream I was driving and took a wrong turn suddenly i was driving down dirt roads and hit some kind of hill throwing my car in the air and ejecting me from it. I get up and looked at my car, again this looked "fake" to me, which allowed me to realize it was a dream and i attempted to fly and do stuff but all i ended up doing was walking down a sidewalk doing as i please, GTA-style. :D

I guess my question is, what could be the reasoning to why my movement is limited, i can now tell when im having a dream (most the time) But there is little i can do, am i not trying hard enough?