Based on my experience recurring sticky pistons can be caused by a few things
Dirt trapped between calliper and piston
Temporary fix – pump the piston out as far as you dare and clean/lube with a cotton bud and brake fluid/silicon spray/fork juice/ etc. Refit reset brakes
Permanent fix – strip clean fit calliper rebuild kit.
Hardened seals on the piston
Temporary fix – pump the piston out as far as you dare and clean/lube with a cotton bud and brake fluid/silicon spray/fork juice/ etc. Refit reset brakes
Permanent fix – strip clean fit calliper rebuild kit.
Calliper Corrosion
Temporary fix – strip clean then very very fine wet and dry to remove the corrosion rebuild the brakes but expect the exposed aluminium alloy to corrode really fast
Permanent fix –new brakes
Top Tips
Before you start;
Read the manual
Watch useful videos on U-Tube or Manufacturers (Formula) website
Have the right tools/bleed kit/fluids/cleaning materials, etc
And make sure you have plenty of; Penitence, Time, Space, Light and in my case Coffee
EDIT
Enjoy 🙂