Genuine Apple Watch strap stuck

I have the Apple Watch Series 2 with Nike Sport wristband (the one with the holes all the way through it). One half of the strap is now completely stuck on the watch. One half comes off easily, but even when I hold down the other release button, the other strap will not move at all. It is the strap that came with the purchase from a genuine Apple supplier. I can only presume that it's because I use the watch for a lot of really sweaty exercise, that I have not removed and cleaned the straps often enough and something has got rusted and stuck. Does anyone have any tips on how to remove this? I have been able to remove the straps in the past to clean, but now, I haven't been able to for several months.