I know that the manufacturer of my printer (Robo 3D) used a commodity power brick adapter instead of sourcing a better one. The original was 19VDC with just enough current for a single extruder and no heated bed.
I sourced and bought a 24VDC beefy power brick adapter with the same plug size and it works much better. The time to heat the extruder has gone way down since it's pushing more power to it.
Connecting a print bed's leads can be problematic, especially since the wires and those connections are using a fair amount of current. This can lead to arcing at those quick-connect points in the circuitry... which then creates black deposits... which raise the resistance which increases the temperature... which increases the arcing...
All this leads to lower heat-up times and loss of power. First things first, (turn off power and) check the connections there at the bed to make sure that they're not arcing and blackened.