Currently, I have some big barrels set-up at spawn and only the jungle and oak barrels open for players while the rest can only be opened as OP. Do you know why this is happening?

Probably too close to /spawn?
Server.properties has a spawn-radius setting set to deny any non op player from doing anything in 16 block radius

Haha...I can't believe I didn't think to check that and that's exactly what it was. Thank you. Cheers.