I actually don't know if this is a software or a hardware question. - and there maybe a simple explanation so apols if so.
I've got an M-AUDIO Keystation 49 plugged into garageband - just for writing. I primarily write and record live on piano/guitar but just recently getting into the electronic side so knowledge is limited. - and I know this setup is basic and cheap so don't hate me. 😀

All of a sudden I cannot play chords on certain instruments. - only one note sounds.
It's obviously the keyboard because the "musical typing" option works fine. (ie playing live through laptop keys)
But midi monitor shows no problems with the inputs from my midi keyboard and garageband is recording all the notes fine and at equal velocity, they just don't sound.

Old songs play back fine...

Bit mystified - please help!

Cheers
Derekthecat