Are there safety features such as the kickstand being down, clutch in or a loose ground wire that effects the start button directly? If so, what grounds do I check on my 2001 Suzuki Katana 600f?
Just check the wires are correctly connected - a mate had issues and I had to redo all the connections as some of the wires are so similar in color and some of the color traces are so thin.
He did have an excuse - he was color blind, we got it sorted though (and as a panel beater he was a magician...)
Even the old bikes can have safety features you mentioned. However, those safety features should not interfere when the bike is put to neutral. (I am guessing that should be standard between all bike models.) Did you try to start the bike on neutral? If it does not start in neutral, we probably are looking for another issue here. Is there anything happening when you push the start button? Is there any sound from the engine?