Different problems, different causes
If SmartThings thinks you left when you are still home, that is almost always a problem with SmartThings, or at least SmartThings struggling to overcome local interference.
If SmartThings thinks you are still home when you have actually left, and you are only signed in on one device, that’s almost always a problem with your phone not reporting its new position, and that will usually affect multiple apps, not just SmartThings. The problem is that as soon as you open an app, your position will probably be updated, which makes it hard to prove that other apps aren’t having the same problem. So in order to troubleshoot this one, you have to set up a non-SmartThings app that also tracks you over time. The free life360 app is a good choice for this and has an official SmartThings integration. If it’s taking a long time to update your position, then to improve things you start looking at your own phone settings and use patterns.
https://support.smartthings.com/hc/en-us/articles/202334244-Connecting-Life360-with-SmartThings
If the problem is that you can’t set a Geopresence region which is working for you, but SmartThings is responding appropriately when you cross the region boundary, then you may need to switch to a different type of device or to a two-device system.
How important is presence to you right now?
I would also note the people seem to fall into two groups when it comes to troubleshooting presence problems. Both are legitimate. They just represent a different current need.
The first group is people who just want the SmartThings official mobile app presence to work the way it’s advertised. They don’t themselves want to spend much time on troubleshooting, they don’t want to look at alternative methods, they just want it to work the way they expected it to when they bought the system. Again, that’s an entirely legitimate position. For them, there are really only two things they can do: contact support and see if support can make things work any better, or try the SmartThings – branded presence sensor rather than the phone.
The second group is people (like me) who really need presence to work now because of requirements they have for their home automation system, and are willing to spend more time and money to come up with something that will work. (In my case, I am quadriparetic, use a wheelchair and have limited use of my hands. I need presence that works, even if it’s cobbled together from multiple systems and devices.) The good news is that there probably will be a way for most people to end up with presence that does work. The bad news is that it’s going to require some trial and error testing, it will probably require additional devices or apps, and it’s going to take time to get it set up right. Again, in my own case, it took about four months to get to the point where I had reliable presence detection. It was worth it to me, but I understand completely that it wouldn’t be worth it to a lot of people.
More information in the presence FAQ: