Periodically, one reads of STM Links being bad. I've never heard of an expert player not being able to play a mouthpiece, it might not be their cup of tea but the piece always plays.
Now, as many others have said the way you put the question leaves no possible reasoned answer, we can only guess what you mean.
1) The mouthpiece doesn't seal well, the cork is too small and in order to play in tune the seal isn't good.
2) The mouthpiece is to open for your chops. Regardless of what it says on the piece.... it might have been open further than your chops can handle.
Most players, especially beginners think that bigger is better. It isn't the size of your equipment....is what you do with it
3) You use a bad reed, Sometimes reeds are bad. I know of people who get one reed out of a box and play....regardless, some others choose their reeds accurately or work on them. Even so, some reeds are unplayable.
4) The reed is too thick for your chops. That is the purpose of numbers or sizes, but sometimes size 3 ain't the size 3 you were always used to or size 3 of brand X isn't brand Y 3 size. Chose the appropriate size for you, not what they tell you is right , but what you can handle. It isn't the size of your equipment....is what you do with it
