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: I am using Overture High Speed these days (2022). It prints in 60% of the time of regular TPU. For instance, a Benchy takes 57 minutes with High Speed Overture…and 2 hours 33 minutes with regular TPU. In Prusaslicer go to Filament Settings>Advanced>Max Volumetric Speed and set that to 4. Filament temperature 240/50.

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) 

You can see all the various sheet sizes here.

Nozzle temperature: 240° C for Overture High Speed TPU (210-240 C for regular TPU) 

(I've gone up to 240°C to get better side-by-side layer bonding)

Bed temperature: 50° C

Print speed: 30 mm/sec with regular TPU. 

With Overture High Speed TPU I picked Generic flex, and only changed to Max Volumetric Speed to 4…no other changes to speed needed! Cool, eh?