sometimes things get stuck in a horn. about a million years ago there was a guy who suddenly could not get his french horn to play. (i read this in a publication called "brass world" which if you remember it, you are as old as i am). he tried cleaning it himself, having it professionally cleaned, taken apart, everything. everybody swore the horn was fine. but the thing would not play right. his day job was being an x-ray tech, so he took it to work and took a picture. he found that a pen knife had become lodged in the bell pipe. no amount of cleaning had dislodged it.
something is majorly screwing with the acoustical response of that horn. if you can play mary had a little lamb on your mouthpiece, it probably isn't you. short of getting your trumpet x-rayed, you could try another trumpet.