my question would be are you making positive firm shifts? If you are just like "doink, I think I'll shift now..tap" then you could very easily not firmly engaging and therefore it isn't going to go in. If, OTOH, you are firmly shifting and it is doing that, i would have some other questions I would would ask.
How many miles are on the clock?
When was the last time the oil was changed?
Is this something that just started happening?
Or was it like this from mile number 1?
Have you dropped the bike on that side?
you might send a pm to hybrid as well