Having trouble with Red minus signs on folders shared to my computer

Here's the situation as best as I can describe it.

We are working on 2 imacs. iMac A as a 1TB harddrive attached to it via USB that has all the folders on it that we use to work. iMac B is connected to A wirelessly and can "usually" access the contents of the harddrive through A with no problems. However, every once in a while I click a folder on the harddrive and I get access (I'm on iMac B) to that folder but the subfolder inside that I need to access (the others are fine) has a red minus sign on it which is infuriating. So far the only way we've been able to re-grant access to the folder for me is to either restart my computer or unplug and replug the harddrive into iMac A.

Steps we have taken:

Added full "read & write" permissions to both computers and users on both the folder, the sub folder, and the harddrive itself

Attempted to set up additional users to access the folder differently

Checked and unchecked "ignore ownership" on all of the above access points

We find it strange that it's only sometimes I get locked out of the folder and other times I'm not. It has to be some sort of glitch unless there's some sort of setting that locks me out if I make too many changes to the contents in that specific folder. Any ideas or help would be greatly appreciated.