My guess is you have a buffer lock on the point preventing you from deleting the value in the snapshot table. If there is such a lock, you can't enter data after the snapshot value unless you buffer the data write. This also means, you are unable to delete the snapshot data as well until the buffer lock expires.
You should be able to confirm this behavior by using PI SMT archive editor to delete the value manual and should see a similar message to confirm this. You could wait for the buffer lock to expire or you could reset the buffered severs list as describe in KB00093