Door locks

[Unleashed]

Hey everyone,
My 1986 190 2.0 has been standing for some time with no passengers being taken in it. It's coming up for its annual check time, but three of the doors don't open from the outside. The drivers side which has been in use is fine, but the other three only open from the inside.
I took the front passengers side lock out, and got the whole latch mechanism. Nothing looks broken inside, but if I pull the external handle lever it just won't release.
Anyone have any ideas? Is there an adjustment of some form that may need doing from it not being used in some time? Just surprising that three doors have all done it nearly simultaneously without even being used!

Thanks!