It's worth checking the jumper bars between the terminals are in place, and making good contact. If so, take out the drive units and check there are no loose connections, and also do the same with the crossover/terminal blocks on the rear.
If all appears to be well, the problem may be in the mid/bass drive unit of the affected speaker: you could check this by swapping over the drivers between the speakers and seeing whether the fault goes away on that speaker. If that doesn't solve it, it points to a failed component or broken connection in the crossover on that speaker, which will require further investigation.
Problem is, if it's a duff driver, you may have some problems sourcing a replacement, so a broken connection or failed crossover component is the easiest to fix in this case.