Hey everyone,
I was actually one of the original people to post a question about issues with the SIM tray not seating properly and whilst I did end up finding out why, I thought I would share my experience and include it with the mix of all the excellent responses that have already been posted. It was hard finding a lot of info online about it initially for some reason and what I experienced also needs to be considered.
I repair phones, computers etc and despite checking the inside of the SIM tray a dozen times, I could not find any reason why the sim tray would not slide into the SIM slot. Even with a decent magnifier I couldn’t see any obstructions and it wasn’t until I removed the metal sheath of the SIM reader which also holds the eject mechanism and sim to the motherboard, that I found the tiniest of shrapnel or what looked like small bits of a broken plastic SIM adaptor. Looks like someone has used a broken SIM tray or perhaps at one point, tried to put a SIM into it without using a tray, breaking it whilst taking it out. Unfortunately in my situation the pins that read the SIM were also damaged so I had to remove and replace the SIM reader entirely on the motherboard to get my problem fixed. (I was lucky I have some soldering skills but I would only recommend experienced repairers try this).
Removing the metal sheath can also be tricky and if not careful, you can damage the SIM reader as well.
Hope this helps