It's NOT MIDI - MIDI gets the blame for everything. The keyboard is faulty. Buggered. Broken. Bust! MIDI is a protocol it is a data stream. It works or it doesn't, like most digital streams. Most usual problems are the data going in at the start and being decoded at the other end. So common problems are note on messages being missed, and note off messages lost. Same thing happens with continuous controller data - levers and wheels. You can inspect the MIDI events in most editors. There is rarely any issues with the data stream. The key issue is getting the data into it in the first place. Remember that MIDI is just switches under the keys, and not even sophisticated ones. MIDI velocity is not a how hard you kit the key, it's the time between it starting to move and ending the move - the quicker, the harder it was hit. If certain notes are missing - then it's a real fault. Some keyboards work by scanning all the keys, putting them into groups and then processing them. Older ones had data lines from chunks of the keyboard that get parallel processed. The internal processing of synth type keyboards also groups things into octaves so C1, C2 and C3 may be shared and then combined with D1, D2 and D3. The exact method your keyboard uses to process key data will be at fault here. You can test the system with a different midi controller - anything will do. If another keyboard adds the missing notes, the fault is the keyboard. I have had corrupt software in the computer cause this, but maybe twice in 40 years? It can be caused by software. It's even a feature of some VSTIs - demo mode or an expired subscription removes notes at random - so certainly it's possible its at the computer end, but you need to try something else first, then you can inspect the data in the sequencer - when you press C3, does it record a C3 note and the issue is you just cannot hear it? Be systematic and work out where the issue is. I bet it's the keyboard though.