Use of thick felt wads tends to open a shot pattern , due to felt wads don't dropout of the way of the shot , once it leaves the muzzle. Over powder wad is all that's needed. Don't forget a thin over shot wad as well.
A few things to consider before blaming the felt wad for blown pattern,
1.)
The driving force (gas propelling the load) dissipates in all directions at the muzzle reducing the driving force on the wad that is pushing the entire load out of the gun, items do not accelerate when they have left the gun.
2.)
At the point the load is exiting the barrel the shot has been forced backward into the wad surface, the complete load is compressed, after it has left only resistance and drag are acting on the load, the wad is light with a large surface area, it is left behind as the shot has weight and momentum. Even shot changes position in flight within the shot column because of mass for a given shot size there are some smaller and larger shot (if you buy 7 1/2 shot not all are that size), the smaller shot slows faster than larger.
3.)
The wad is often seen with the eye because of it's size ( I'm not talking plastic shot cups) but can be misleading, an example is watching somebody shoot a clay target and when they miss state that they shot behind because they saw the wad, if a breeze was present the wad may have drifted to the side, the reason they missed was more like they stopped the gun, wads go in all direction not always inline with the shot.
4.)
Consider that some people are referencing "
a felt wad" not a thick fiber wad full of wax, even heavy wads have are subject to the same physics if the shot volume (weight) is greater, if a wad (felt or fiber) is loaded on the powder then a round ball lighter than the shot load in many cases topped with a card to hold it in place , when shot why does the wad not knock the ball off line, just a thought.
5.)
If the pattern is being blown I would look at lowering velocity, make some fiber wad with paper shot cups attached (not twisted at the front end), shot and card on top, with most brown paper as a cup and a load not over fast the shot will not rub the bore deforming the shot.
Pictures show set back in plastic cup and fiber wad with card over, shot has gone. Just my 2 cents after doing a 3,000 round test a few years ago.