I have 1 bobbin set just for each size of thread i tie with... I put a spool of thread in a bobbin with the leggs of it bent closed before putting on the spool... I pull on the thread and if the thread breaks i take out the spool then slightly spread the bobbin leggs open... I repeat the process till the thread doesnt break any more... that fixed the problem i used to have with thread slipping on the hook shank... You want your thread to be just tight enough that it is really close to breaking... Another cause for the problem your having might be the thread in general, if its a prewaxed thread the wax dries out after a time and can become slick causing a slipping problem...