About 6 months ago I bought some 4s batteries and flew them a handful of times before some of them were no longer being detected by the charger or the voltage checker on my charging board when plugging in the balance leads . I then tried plugging them into a drone, but nothing happened. I assumed that I had damaged the leads while flying (even though there was no visible damage) and bought another set. Now I am having the exact same problem with some of the new batteries (which are a different brand ) and I don’t know what I am doing wrong. What might be the issue here?  could it be a problem with my charger ? did I just get unlucky with my batteries?

The new batteries don't work with the drone?
I would have assumed that you accidentally pulled one of the wire leads of the battery, but that it looked okay because the break was underneath the heatshrink tubing. But if the new batteries also aren't working...I guess I would read up on Lipo batteries to ensure you have a good understanding of the oddities of these batteries and that you're not doing something unintended like fully discharging the batteries, then I would look at my battery supplier with skepticism.
Good luck!

The normal absolute minimum voltage for a LiPo battery is 3.0v per cell.  If you have discharged them below that condition most chargers, and some checkers etc will not register them as having voltage at all.

Some chargers will refuse to charge if even one cell is under 3.0v.

You say you plugged them into the drone and nothing happened. Is this nothing as in totally dead no lights on the drone, or that there were LEDS lit on the flight controller board but it would not turn props/fly?

You need a multimeter and then carefully (outdoors away from property) test each cell by prodding the balance plug connections. DO NOT SHORT/TOUCH TOGETHER the connections/prods. Number the batteries and table results.

Come back and inform of results?

When I plugged them into the drone there was no lights, nothing . when I went out to test with a multimeter the voltage in the main lead for all of them was below a volt(exept one). I attempted to fill out a table of the cell voltages but they are so far out of the range of what they should be I am thinking it might just be noise . I also tested a working battery with my multimeter and it seemed to be calibrated fine.

   

it seems they are all dead.

they shouldnt went under 3v per cell.
store them on 3,8v per cell is benefical for storeage lipos.