It looks to me like a lot of your branded shirts are attacked by mobs and mobs of fake sellers, selling at a fraction of what you are.
They are most likely scammers that get reported and removed in a day, but not before they have siphoned some sales, often by leading gullible customers off Amazon to complete the “sale” and then they never ship.
Read the many threads on what is going on. If you are being attacked,you are going to have to spend time every day reporting these thieves untl Amazon finds a way to shut them down.
Gangs of them (or maybe the same seller with multiple IDs) come on at once, with American sounding names and somehow claiming to be shipping from the US but most are overseas.
Also your feedback is not the greatest and in a lot of cases you don’t have the buy box.
But I think the bloodsuckers are your worst problem.