MK2B heated bed only half heating

[printbus]

RAMPS D8 is protected with an 11-amp polyfuse. Overcurrent will cause the fuse to heat and open. When it cools, it closes and applies power again. MK2B is spec'd as 1 to 1.2 ohms in 12V configuration. Ignoring wiring losses, this is 10 to 12 amps. You're likely fine especially after allowing for resistance in the wiring from the power supply to RAMPS and then from RAMPS to the heat bed, and for the on-resistance of the MOSFET switch itself. Do make sure the power supply, if adjustable, is set for 12V and not something like 13.8V.