sounds like you aren't getting the tip to the surface before the cast. 2 tips:
1) Strip the fly and tip to the surface before the lift. If you shoot a few feet of line on the forward cast, then you can strip a few feet of line back in after the swing, immediately before starting into the lift. This bring the tips to the surface, where it is easier to move them
2) Start the lift slowly, and don't apply power until the tip is about the exit the water. When lifting sinktips, especially with softer rods, be very cognizant of the moment immediately before the tips exit the water. That's when you want to begin applying power and start your casting stroke. If you apply power too soon, right away when starting the lift, all you do is put a deep bend in the rod. Lift with a slow start, begin to apply casting power right when the tip is about to exit the water.
I hope these 2 tips help. good luck!
CF Burkheimer fly rods advisory staff
Nextcast lines pro staff