Flexible filament can stick too well to the bed of the 3D printer.
Here is the breaking news! NEW DISCOVERY (as of 2016)!
Putting a little talcum powder on the bed solves the problem!
By the way, the surface of my printer bed has a peel and stick sheet called PEI. I love it. Obviously if your printer has a different surface you have to experiment to see if powder works for you.
The PEI surface becomes "tackier" when warm, but also softer…so I let it cool before going at it with a flexible putty knife! Before printing I clean it with some alcohol to remove any invisible trace of grease from my fingers, then I add the powder for flexible filament prints.
I am putting less and less powder each time. It really only needs a tiny bit.
Talcum powder: Baby powder (with no perfume, yay!)
(I'm using the kind with talc, not corn starch…they may both work…I dunno yet)
Filament: Flexible TPU 1.75 mm
Printer: Original Prusa i3 MK2, MK3
Bed surface: PEI peel and stick sheet (MK2 260mm x 225mm or larger; MK3 260mm x 242mm or larger)
Nozzle temperature: 210-225 C
(I've gone up to 240°C to get better side-by-side layer bonding)
Bed temperature: 50° C (I am still experimenting with this, too)
Print speed: 30 mm/sec
I've also tried 40, 50 mm/sec…yes, I'm still experimenting with this, too
A little talcum powder makes it easy to remove sticky prints
Here is video comparing a half bare bed vs half with baby powder