Sonoff SNZB-04, is it me? - or do you get what you pay for?

I have a bunch of SNZB-04 Contact Sensors. I know they don’t cost much so perhaps I’m expecting too much.
Mine pair up ok, they stay online most of the time, although they are not perfect but the real annoyance is how they intermittently fail to respond when a door/window opens.
Take for instance the one on my bathroom window which should cause the extractor fan to turn off when the window opens. 20% of the time it doesn’t ! - If I go to the window, close it then re-open it everything works correctly.
Not the end of the world but annoying !
Anyone else had any experience with these units ?
Thanks

Do you have any mains powered ZB devices in your network?

Bucket loads ! - Over 100 devices in the network, most of them routers.
I don’t think this is a signal problem as its only the SNZB-04 devices that behave like this.
I’ve also tried putting QoS to 2 but it doesn’t seem to help
(I’m using Z2M by the way)