The Snugride is a very common infant seat to see installed on the plane, but that's not to say that on a particular plane you may not have trouble -- the seatbelt configurations show a fair amount of variation from airline to airline and even plane to plane.
I once was on a flight where the "male" end of the seatbelt, the length of which was not adjustable, was falling precisely at the beltpath opening on my convertible carseat. This might be similar to what you're describing. The only thing that worked for me was to push the carseat very hard down into the airplane seat to compress the seat cushion a bit -- this allowed me to get the seatbelt to buckle inside the belt path instead of right at the opening.