Heat bed taking a long time (no replies)

I've been having trouble getting my heatbed up to the right temps. Initially the problem seemed related to running the printer through Pronterface. Once I started printing directly from an SD Card, it was heating much quicker. Then I changed the settings so the bed would heat to 90 for the first layer and 100 for additional layers, seemed to do the trick. But yesterday after I was done calibrating the bed and the extruder, the heatbed started taking a long time to heat again. Right now I'm heating it and it's been 32 minutes and only at 85 degrees. I checked the wires, everything seems fine. The thermistor is insulated. I'm not sure what the problem could be unless the power supply isn't good enough or maybe I'm using the wrong electrical sockets?

I have a MakerFarm 6" i3 with the Magma Hot End and my power supply is the Antec ATX 12V, v2.3, 450 watts.


Thanks!!!!