Make sure the paper is in the bay correctly, it should feed under and up so that the paper comes through the slot without bending
Hold down the button on the top, then plug in the printer to power. It should print out a test page
The green LED will not be on solid, it will blink once in a while, that's normal.
Make sure you have a good power supply that can deliver the required current. If you are trying to operate at 5V, your power supply must be able to maintain a constant 5V even when the current demand spikes during printing. If your printer is making an attempt to print, but then struggling, then it is likely your power supply is dipping below 5V.
If possible, try a higher voltage power source. At 9V you should be able to get away with 1A.
Make sure the paper roll is not 'stuck' feeling in the bay, it should rotate easily!
Some changes have been made to support a broader range of Arduino-like boards. Older code will require updating. Fortunately it's just a few lines around the global declarations and the setup() function.
Adafruit_Thermal printer(RX_PIN, TX_PIN);
New syntax: declare a SoftwareSerial object and pass its address to the Adafruit_Thermal constructor, like so:
SoftwareSerial mySerial(RX_PIN, TX_PIN);
Then, in the setup() function:
The printer's features and behavior have changed over various firmware releases.
First, check the PRINTER_FIRMWARE value in Adafruit_Thermal.h, make sure it matches the value at the bottom of the printer test page (hold down the paper feed button when connecting power to print out a test page).
Some features just behave a little differently among releases…barcodes, line feeds, etc.…if you were previously using an older library or an older printer, you may need to tweak the code to produce the desired formatting with a current setup.