perhaps your firmware does not use these commands?
did you download marlin from the original website or developer?
did you change any command codes?
you could always add something at the end of the print like
G1 X0 Y0
but my prints are ending like this
G92 E0
M107
M104 S0 ; turn off temperature
G28 X0 ; home X axis
M84 ; disable motors
stupid question,
is your heat bed connected to your ramps?
or powered independantly?
you could try with another slicing program.
What are you using, in use Slic3r because i have the greatest success