There are many different things that can cause these issues, some in the cloud, some in the house.
First regarding where to put the hub…
The SmartThings hub is a single plastic box that contains four different devices: an ethernet connector, Zigbee Coordinator, a zwave controller, and a Bluetooth radio (currently inactive).
wiFi, particularly boosted Wi-Fi, is in the same frequency range as Zigbee, and consequently can drown out the zigbee signals.
For this reason, you want your SmartThings hub to be at least 3 m from your Wi-Fi router (not your ethernet router) as well as any Wi-Fi booster. Yes, many people do make it work when they’re closer together, but it will be easier if you create this additional space.
Ideally it should also be in the open air. Definitely not inside a metal cabinet.
If you move the hub away from your ethernet router, but plug it in 4 inches from a Wi-Fi booster, you may actually make the interference problem worse.
At my house, we do use a Wi-Fi booster. If I plug it in on the west wall in one room, all the zigbee devices west of it on my SmartThings network stop working. If I just move it to the north wall in the same room, they’ll start working again.
So the first step is to try to get the hub away from any Wi-Fi broadcast devices.
Once you’ve done that, you can run through the standard checklist for contact sensors.
If that doesn’t help, it may be a cloud issue. You may need to enlist support to help run diagnostics from their side.