Since you probably bought a cheap chinese proxmark instead of the expensive ones, you might wanna test basic functionality.
hw tune
125.00 kHz ........... 25.73 V [+] 134.83 kHz ........... 17.49 V [+] 121.21 kHz optimal.... 26.12 V
[+] Approx. Q factor measurement [+] Frequency bandwidth... 6.4 [+] Peak voltage.......... 7.6 [+] LF antenna............ ok
Something between 20-45V would be perfect.
13.56 MHz............. 16.09 V [+] [+] Approx. Q factor measurement [+] Peak voltage.......... 4.7 [+] HF antenna ( ok )
I didnt find as much info here, but it seems 6-36V is all good.
Now test how you should be placing your tags on the readers.
hf tune -> Move the card around a bit. Once it drops to 12-13V, thats your optimal placement.
lf tune -> Do the same as above and search for the lowest voltage you can get. I dont have any 125khz tags so I dont really know what optimal voltage would be.