It sounds more like there is uneven contact between the rod piezo pickup and the bottom of the saddle.
Being that it was used, you do not know if it was manipulated by the previous owner, however it is easy to see what is going on.
If you slacken the strings enough, you will be able to remove the saddle (white plastic-looking thing that the strings ride over before the turnaround at the butt-end, after which the strings go to the tuners...
Once you remove the saddle you can inspect it to see if the bottom is perfectly smooth and even.
Then continue on, and use a toothpick or something similar and you should be able to gently pry up and out the rod piezo, but be very careful not to bend it much at all at the end where the cable goes into the bottom of the saddle-slot in the bridge, for this is easy to break if you pull it out incorrectly, the best way is to actually pry it up from the end that has the tiny cable, but you need to fiddle with it to see...
Then inspect the rod piezo element, make sure that the top is even, or if it has 4 'bumps' on it, that they are all the same height (digital calipers can help)...
After checking the above, both SADDLE and ROD PIEZO, if you can report back what you find, BEFORE re-assembly, further advice can be given...
Please write back with the details, and also closeup photos can help if you see anything odd.