two parts to your problem. first part is practice which i'm sure you already know or you would not be taking the time to email me if you did not care about improving. second, bottom wrestling is like riding a bike. the reason i refer to riding a bike is because once you understand how to maintain the position on bottom it really does not change. once i understood how to hold my base and seal off it never changed and I used if from little league all the way through college. this is the exact reason why we have a riding time point in college because nobody can hold good guys down.
here are a few things you need to work on and then advance from here:
you need to work on being able to hold your base. first moves off the whistle are great but when you don't win on the whistle you need to know how to hold your base, seal off, get hand control, and then get out.
base drill: get in a base and hold it for 30 sec. or 1 min go's where the top man can do whatever he wants to try and get you out of it. this will force you to learn how to shift your hips against his pressure. see video of base drill
sealing off drill: top man attempts to put legs in, pull you out of your base, or attempts to tie up a wrist. your job is to keep the legs out (keep hips low, knees together, sometimes shifting away from him) and keep your wrist free. see video of sealing off drill
capturing hand control: work on shoving your opponents hands from under your arms to the outside of your hips and keeping control of his wrist without getting your wrist tied up. make sure to keep your elbows tight to your body.
once you feel you have this then you add your stand-up, sit-out, or whatever else you like.
best of luck