Batteries tend to have a finite lifespan.

Consider them to be like a fruit or vegetable which have limited shelf lives.

NiMH batteres will self discharge over time and that can affect the ability to recharge the packs fully.

People who have been in this hobby for a while learn to test their NiMH receiver packs at LEAST yearly to check for decreased capacity or other problems.

So it is likely you are on the right track.

Change out the battery packs with a freshly charged new pack and see what happens.

Then if you know anyone with a good computer controlled charger with capacity and discharge testing capabilities, let them test out your old pack for you.