Changing K-drill blades

Realized my blades were extremely dull last week, so got a replacement set. I was able to get them off, but I can’t seem to get the center blade back in, anyone change it out before? Any recommendations?

I have not changed mine out yet but not sure what might be the issue. Check for a burr on the new blade where it enters the slot?
https://3989ac5bcbe1edfc864a-0a7f10f87519dba22d2dbc6233a731e5.ssl.cf2.rackcdn.com/vexilarnew/pdfs/Kdrill_Manual.pdf