Only came across this now.
This is a common enough problem with a badly adjusted gear lever and easy enough to check and for some strange reason effect 2nd to 3rd gear mostly.
Ride the bike and when you are in second gear move your foot away from the lever completely, then return you foot to change gear again.
If it goes in every time you do this then the lever is adjusted to low
The problem occurs because the lever has not returned completely to the home position as your boot is holding it slightly off the home position and when you go to change again you get the blank as you describe.