Heat Pads - dont think they work good!

We've had our heat pads for over two years but I dont think they work as good now ..............anyone else finding this?

It used to once they were done in the microwave I could barely touch them to fit them in to the covers but now I can grab them for long enough.
Last week before I went to bed I put them in with the bunnies, an hour and half later Leon starting thumping (he likes to wind us up), my boyfriend went to check on him and he said the pads felt cold I never thought they ever lasted 8 hours like it says on the box but and hour and half is just ****

Do you have to re-new them evry so often?

Yes they do have an expiry date, I forget how long it is though. Does it say on the label? I guess it depends how often they're used as well, if you use them a lot then a couple of years sounds about right.

Mine tend to become distorted, I'm not sure why. But I've had to replace them quite often so haven't run into your problem :?