My guess (just a guess) is that you’ve ended up adding Devices in a different sequence than previously and a different selection of parents is now available to the door sensors than when they were originally paired, resulting in some child slots not being available this time around.
If so, this should be fixable, but can be very tedious to fix. 
For example, from a troubleshooting aspect the surest approach would be to remove all zigbee devices from your network, then start by adding one of the troublesome door sensors near the hub.
If it pairs clean in that setup, it’s very likely a child slot issue, although what your next steps would be, given that SmartThings doesn’t provide any mapping utilities, could involve a lot of additional trial and error. 
If the sensor won’t pair even with no other zigbee devices except the hub, then it’s a different issue.
Anyway, the Xiaomi devices are very cheap, but they are only intended to be used with their own Gateway, so they can be idiosyncratic. 
I don’t know how much effort you want to be put into this, but if you do want to start troubleshooting you will need to be very precise and patient, and keep notes as you go. A lot of work, I’m afraid.
To start, give us a list of every zigbee device you have (brand and model).
Ideally if I were troubleshooting for myself I’d also Make a floor plan showing positioning but for security and privacy sake it would be a bad idea to post that to a forum. But if you want to do one for yourself it would probably be helpful.
Meanwhile, read the following FAQ as it will help explain why I made my initial guess.
@a4refillpad or @veeceeoh might have some additional thoughts.