Inconsistent reading

[neurochill]

I am having issues using this code with a Raspberry pi 4 and a Grove 125kHz reader. Before actually reading the tag, the first few attempts result in many printed strings looking like 'x80\x00\xf8 etc.
After some time and some attempts, the RFID ID does get read, but then never again. The system seems to fail to read it again.
Has anyone else encountered this issue before?
Thanks for any help!

[rgrokett]

What type cards do you have? Ones that came with the Grove reader? Some cards have extra data sent, besides the id#.
Do you lay the card onto the reader for a second or two?

What gets printed using the rftest.py program? This is just a simple serial port reader but is currently set to only read the first 12 bytes, maybe change string = ser.read(12) to string = ser.read(40) and see if the card is sending more than 12 bytes.