Hey there - I've no picked up two new membranes for my a1200, and I have the same issue...which logically means that the issue isn't the membrane...
All the keys work except the left-shift, left-alt, and ctrl. Seems they are all on a similar matrix and go to the penultimate pin on the right of the ribbon cable. Have tried cleaning the connector as well as I can. Next step might be to look at the bottom of the motherboard and be sure all the pins look good. Perhaps I need a new keyboard connector on the motherboard?
However, if this is a common problem, (I have seen it mentioned before in passing) I thought I'd ask here and see if anyone has come across a solution, or any other ideas.
Thanks.