You can try an use a rubber mallet? also run the bike till the pipes get hot and see if it'll slide in... I meant heat up the inlet so that it expands just a bit to slide through all the way. That might be better than the mallet idea.
Measure how much room (length) you have on the inside of the inlet and compare that to how much there is on the mid after the welded on ring. They may have made a change in length, and if those aren't the original cans/mids set then that might be the issue. I think there is probably just something in there no letting them seat.
and if those aren't the original cans/mids set then that might be the issue
That happens all the time with Sato exhausts. They constantly made changes to their product lines with revisions that made their products better and when they did that they would retool all their product lines to the new specs unlike some companies that churn out the same flawed products for decades at a time.
The downside is you end up with a lot of different styles of cans, inlets, spring points etc and once the parts are discontinued as they have been then people start mixing & matching parts from other models to make it fit.