Left AirPod drains faster than right

I recently bought a set of AirPods for my wife and until two days age they charged/discharged equally but now the left airpod discharges much much faster when being used than the right.

The last two times she has used them for ±1.20 hours and after this time the left airpod is @17% power and the right is @75%!

When she charges them again they both charge back to 100% at the same time.

I read a post where it suggests draining the left one to 0% then continue using the right one until it's also at 0%

then recharge as normal to fix the problem but this has made no difference, the left one still drains faster.

I was hoping someone here might have another suggestion as to how we might fix this problem?

Thanks in advance,

Dave...