I know your post is 13 years old, but it got me thinking. Usually we QC out Anti-C3 reagent with the tube method, but I was curious to see what would happen if I used buffered gel card instead. This is what happened:
1) I converted my 3% C3 control cells to 0.8%.
2) I pipetted 50 uL this 0.8 cells into 2 wells of the buffered gel card, one labelled "pos" and the other "neg".
3) I pipetted 25 uL of Anti-C3 into the "pos" well.
4) I pipetted 25 uL of saline into the "neg" well.
5) I centrifuge the card for 10 minutes.
The results are as expected. The "pos" was 4+ and the "neg" was 0.
This was a stronger positive result compared to the tube method, in which we usually get is a 1+ to 3+ reaction.
My conclusion is that maybe there was something wrong with your gel cards. Did you use MTS diluent or saline when you converted your cells to 0.8? I am not sure if that would matter, but considering your samples were weakly positive to begin with, maybe the conversion "diluted" the samples.