iPhone 8 wireless charging problem

I have an iPhone 8 and an Anker Qi wireless charger (model AK-A2513011) and am having trouble getting wireless charging to work consistently.

When I first place the phone on the pad, the lights come on and it starts charging fine, but if I return a couple of hours later, the charge has increased only slightly, so I replace the phone and repeat. Overnight the phone never reaches 100% charge - charging appears to be interrupted for some reason. With this charger, the indicator lights come on for a few seconds to indicate initial good connection, but then go out, so it is not possible to check if charging is continuing other than by waking the phone. My son, who has an iPhone X, has tried it and it works perfectly, so it's not a charger problem.

I might add that I have the phone on 'do not disturb' overnight, which should eliminate any vibration I think (?). I've seen shifting position owing to vibration as a suggested cause.

Has anyone else experienced this? Any ideas please? Thanks.