A lot of mouthpiece problems originate here- on the table. Sometimes it is easy to spot, as with this high-end soprano mouthpiece from one of the well-known makers. Look at the picture.
What do you see?
Well, you see the area I circled, I hope, and if you do, you see a very worn area right at the window-end of the table. But the rest of the table isn’t worn that way at all. So, what is up with that?
It’s simple. The table is not flat, so the reed does not firmly seat on the table. Sometimes it makes contact with that area near the window and sometimes it doesn’t.
As in: any given moment of any given note on any given song on any given day. In other words, randomly That intermittent contact caused that wear-and-tear as shown.
Now, this mouthpiece came to me because a player could not get it to play in tune. Did it play flat? Yes. Did it play sharp? Yes. It did both, seemingly whenever it felt like it, note to note and even during one single note.
That table was the cause, pure and simple. But…. how did the table cause both to happen?
Here’s how: if the player used a relaxed embouchure, the reed was not seating firmly at the window-end of the table. That, in effect, created a slightly larger chamber. When that embouchure was used to tune up before playing, the mouthpiece would go on the neck a small amount more to accomodate that chamber size. It might be so small an increase as to not be noticeably different. When the performance started and the player might increase the firmness of his embouchure even slightly, that portion of the reed would be forced down onto that area of the table and then the chamber, in effect, was smaller. And then the pitch would go sharp. ( Add to that  the effective facing length was also shortened, which adds slightly to the increased sharpness of pitch).
If the player tuned up with a firm embouchure, the opposite would occur. The mouthpiece would not go onto the neck quite as far. But, when the player relaxed the embouchure at all, the reed would recede from the front of the table, vibrating against that spot as it did so, and the chamber would, in effect, increase in size and then, the pitch would go flat. (Add to that the effective facing length was also lengthened, which adds to the increased flatness of pitch).
Once the table was flattened and the correct facing restored, the piece played fine. No change in tone at all but much more responsive and, can you believe it, it stopped being reed-picky, too. And it plays with solid intonation.
Maybe you should look at the table on your mouthpiece once in a while.