Not saying this is definitely the cause in these cases, but another thing to keep in mind is that Clock accuracy is crucial in getting sync to work correctly between devices.
To illustrate, if you somehow create an item when computer your clock is set to +1 year from now (2020), that item will not accept new changes from other devices until 2020 because it’s considered to have “newer” data already.
Another example: Let’s say you sync now, then rewind your clock back a few days, create an item, then correct the clock and try to sync. The new item will never get synced to other devices because it’s considered synced already according to it’s creation date, which looks like it was after the previous sync.
Issues of this kind can only be solved by having the same clock on all devices, or more realistically, just setting the correct clock value.