Whilst the thermal printers as mechanically very simple - the drive circuitry for the printhead is crucial to correct operation. Compounding the problem is the high peak currents - this makes the pcb layout critical. Also there can be a few rules to the on timing for the dots- read the datasheet carefully and test your code with an oscilloscope rather than the printhead - otherwise you'll zap a few printheads! See if you can get a complete thermal printer assembly to see how they solve the problems of the drive etc - this may give you a head start. The epson print mechs have good datasheets - it maybe worthwhile to check them out.