I had the same problem fitting mine onto the carbs. Problem as I saw it was the plastic body holding the trumpets pushed in and became concave when trying to push on the middle two trumpets, leaving them stranded.
I removed the four trumpets and because the body is plastic, I used a heat gun to warm up the face where the trumpets fit and from then inside, pushed the face outwards, focusing on the middle two trumpet holes.
When suitably convex shaped, I then dropped the plastic body into a bucket of cold water, to freeze this new shape.
Refitted the trumpets and then the complete assembly back onto the carbs which gave a better, longer fit on the middle two trumpets.