Ron, not all 8 mm collets use the same THREAD PITCH as well as the same THREAD DIAMETER and THREAD PROFILE. Those are often the reasons for an incorrect fit.
Don't try to force the drawbar on to the collet if it feels tight. You can ruin the thread.
As usual, this information is in the books. I suggest you consult De Carle's book on Watchmaker's and Model Engineer's Lathes. The book can be bought for less than a good drawbar!
Thread diameter and profile are likely the most noticeable differences. The thread on Boley collets is the smallest diameter amongst 8 mm collets. Use an accurate Vernier caliper to check for differences in the collets you have. The thread profiles also differ. Some owners may also have also tried to retap their drawbars and/or collets, making the situation more confusing and possibly ruining their equipment.
Some of the replacement drawbars have also been threaded for a "somewhat universal" fit and will work with most collets but if you have original drawbars and collets, some of them will not work with each other even though they are all 8 mm - and I am not talking about the Mosely long 8 mm collets.
Other more knowledgeable people have discussed this on the MB and elsewhere on the Internet. Do a Google search. Here's a good one for STARTERS
In my somewhat limited experience, while some of the other brands also seem well-made, Levin equipment seems to work the best. They seem to have been made the best and they also seem to accept more 3d party collets than any of the other brands of lathes. I guess you get what you pay for because Levin equipment can be very expensive. If I had the money I would just stick with good Levin equipment and be done with it.