If the problem is that one of the tabs won't disengage, it's very unlikely the problem is a screw or anything like that holding the case down.
I've found IIsi cases are really stubborn this way. I've also found that many 68k Macs - particularly IIsi's, as well as IIci/IIcx/Q700s - have this same problem where the tab on one side (usually where the PSU is) is much more stubborn than the one on the other side.
My hypothesis is that heat differentials in the left and right sides of the machine, and the plastic getting more brittle as it ages, combine to produce this problem.
Unfortunately I would say you're going to have to keep gently but firmly trying to get that left tab to lift until you hear or feel it "pop." Once that happens it may still be tough, but at that point you can giggle, shimmy and other wise push and nudge the top case around until it starts to come off.
It's only happened to me once or twice in about 100 cases, but there is a very small possibility the darn thing might not open up unless you break the tab.
M
I've found IIsi cases are really stubborn this way. I've also found that many 68k Macs - particularly IIsi's, as well as IIci/IIcx/Q700s - have this same problem where the tab on one side (usually where the PSU is) is much more stubborn than the one on the other side.
My hypothesis is that heat differentials in the left and right sides of the machine, and the plastic getting more brittle as it ages, combine to produce this problem.
Unfortunately I would say you're going to have to keep gently but firmly trying to get that left tab to lift until you hear or feel it "pop." Once that happens it may still be tough, but at that point you can giggle, shimmy and other wise push and nudge the top case around until it starts to come off.
It's only happened to me once or twice in about 100 cases, but there is a very small possibility the darn thing might not open up unless you break the tab.
M