First a CAVEAT: I don't like Trailboss. Never did. Of course, now that I don't shoot any Smokeless at all, I like it even less if that is possible. Now the Important stuff:
Straight Wall cases "normally" will not expand enough to seal the chamber. This tendency is most likely to manifest itself in Rifles with Rail Road Tunnels for chambers. Handguns, for some reason, seem less prone. Trailboss also burns somewhat dirty.
With straight wall cases, the only positive way to eliminate Blowby is to anneal the cases. Making the cases softer allows sealing the chamber. The other thing worth looking at are Chamber Throats. After first slugging the bore, then check the throat diameter. It won't help to be shoving a .452 bullet thru a .448 throat (Big problem with Ruger). An undersize throat acts a bore obstruction and will increase felt recoil and cause the chamber to pass gas.
You may have been getting blowby before but the propellant you were using burns cleaner than TB.