Product inventory hangs a lot of the tme when adjusting stock manually. We have to try doing it several times for it to take effect, does anybody else have this problem or a solution....
This is likely to be a time-out during import of the data. Depending on your server's setup it could be a number of places that the import is timing out.
The most common is in PHP. You can change the maximum execution time in a number of ways explained here on SO.
If you are using Apache ignore this. PHP might be happily importing but nginx will give up. This page explains how to fix this.
I mostly don't deal with this, but if you are using apache and changing the php.ini settings (and restarting apache) does not help, then perhaps there are other settings.