Motion sensors going unresponsive

I have 5 hue motion sensors
No hue hub
They connect via the zigbee hub in my echo plus

I use them with alexa routines to switch lights on and off.

Until recently all was fine then a few weeks ago the sensors started to go unresponsive at various times

I have replaced batteries

This morning for example sensor 4 and 5 were unresponsive. I reset them, aleza discovered them and I added them back to the routines.

This afternoon sensor 5 is ok but 4 has gone unresponsive again. The other 3 are still connected.

I did buy a hue hub recently but it didn’t improve anything so I sent it back.

Can anyone please help me with this?

Thanks