Can you see the wings of fly moving in real time with your eyes? The answer is no.
With any mechanical connector where movement is possible, like the expansion port edge-connector, it is possible for movement to cause one or more of the contacts to momentarily break for a very short time. Far too briefly for a human to notice, but long enough to affect the electronics.
PCB cards with special “edge-connector” contacts were originally designed for rack mounted systems, to mate with a edge-connector socket mounted at the rear of the rack. The cards fit into slides at the top and bottom, or on either side. And are often held in place with a screw or other mechanical fixing so that they cannot move. These are very reliable.
But on a home computer, where there are no mechanical fixings, the edge-connector ends up being a pivot point.
Blu-tack is good. A long strip of foam backed, double sided sticky tape is better
![Wink ;-)](./images/smilies/icon_e_wink.gif)
Oh, and the edge-connector contacts need to be completely clean.
The Sinclair 16K RAM pack was worse, due to its shape and dimensions.
Mark