Printer heats up but doesn't start printing
Marlin firmware runs a test to determine whether the extruder temperature has stabilized, before it starts printing. If your printer heats up, but never starts printing (or seems to take excessive time to start printing after the extruder reaches temperature) it may be failing the "Temperature Residency" test.
From the Configuration.h file in Marlin:
#define TEMP_RESIDENCY_TIME 10 // (seconds) #define TEMP_HYSTERESIS 3 // (degC) range of +/- temperatures considered "close" to the target one #define TEMP_WINDOW 1 // (degC) Window around target to start the recidency timer x degC early.
Once the temperature is within 1 degree of target, it starts a 10 second timer and if it sees more than 6 degrees variation in that window, it tests again, and again...
If you can observe significant temperature variation, it's highly possible this is what's preventing the print from starting. The best fix is to stabilize the extruder temperture by tuning up PID values.
Or, to quickly determine whether it is or isn't the Temperature Residency test that's holding up the print, edit Configuration.h to give a shorter time window and a larger Hysteresis window (and reflash/upload to the printer). For example, try changing TEMP_RESIDENCY_TIME to 5 and TEMP_HYSTERESIS to 5. That should let it pass the test unless temperature variations are extreme (in which case you try can relaxing the test parameters even more.) It's not advisable to continue printing with extreme variations, but you may at least be able to isolate this as the cause of the initial failure to print. You can also increase TEMP_WINDOW to 3 or so to let the testing start before it is quite up to full temperature.
If this solves the initial problem (of not printing), it probably means your extruder temperature is not well regulated and PID values should be updated. See http://rigidtalk.com/wiki/index.php?title=PID_tuning
For info on editing/updating firmware see http://rigidtalk.com/wiki/index.php?title=Firmware