could be a defective kickstart assembly.(at least the part of it that moves the gear on the shaft into position; or the gear itself) i would take the crankcase cover off and take a look at your kickstart assembly and the gear that the kickstart gear comes in contact with. my guess is that your gear on the kickstart shaft is worn down heavily.
is there any resistance when you go to kick it and it "goes straight down "?
or if there is some resistance, does it feel like the noise that its making(if that makes sense)?
it definitely is a problem too, your kickstarter should work every time. plus ANY grinding on your bike is HORRID!!!
also if your hearing a grinding, you might not want to ride it till you check your oil, odds are its gonna be greyish and metallic which is pretty bad for an engine.