Problem closing apps by swipping up

I have a XR iPhone running iOS 14.3, I purchased over 18months ago.

I have been periodically closing all my apps together by swiping up till they are tiled then 'flicking them up and off' one by one till they are all closed.

In the last week I have only been able to do this to around three, then the 4th one won't move. Then I log out and start closing them, however the 4th one won't move again. This goes on ...

What can I do about this? OK I can live with the problem as I can close things one at a time, but why have things changed and what can I do?