Shoulder Muscles Locking????

tobnrn

Hey lifters. Got a strange thing happening lately. I've been lifting heavy & gaining in all my lifts except shoulder presses. I can do 27.5 lb DB shoulder press. I know I have the strength to do 30s but when I get halfway up my shoulders lock & wont go up. If I go back to 27.5 they go no problem & feel to lite. I can do way more than 5*5 @ that weight which tells me there is no reason strength wise I shouldnt be able to do at least 1 rep @ 30.

Any thoughts as to why my shoulders would lock up. I cant even do a 1 rep max because of it. Very confusing.