Originally Posted by Willem
Getting better ankle flexibility will take years I think.
When I practise with fins my kick feels a lot better, but as soon as I remove them, my legs sink and I my kick isn't good enough to keep my balance, however I keep my head straight down and my leading arm quite steep for my balance. So I suppose my weak kick is the main problem.
Maybe the problem is not so much about your kicking, but rather with your balancing awareness. You mentioned that your kick feels a lot better with fins, because fins do provide better forward momentum and also a bit more buoyancy. If you are solely practicing your drills and swimming with fins, I would suggest that you take off your fins and begin doing those basic drills again, specifically those deal with balancing issues.
I too experience exactly the same as you did -- feet sink last year. Also your ankles could be stiffer than you think, so when you are doing flutter kicks, make the motion of the kicks come from the rotation of your core through your hips rather than kicking like you are running in the water. Problems with using leg muscles is that, you tend to dorsiflex your foot, which then acts as a water scooper which can cause your feet to sink or swim backwards!