Bilstein Shocks And King Spring Setup

[nisskid]

as i was putting my suspension together consiting of the GB Bilstein shocks and king springs using the stock strut top, i lost one of the little dark nut things that sit on the ridge on the shaft (roughly 10mm-15mm in diameter) so i decided not to use one on one of the rears, now i didnt think this was a problem at the time as it had a large washer that sits on that ridge for both sides, and that all that little nut thing did was sit inside one of the bushes when compressed so it wouldnt make any difference.

but as i drove the car and went up a windy road (straight after it was installed to test it) i noticed when i got to the top it was alot lower on one side, i just took it slowly on the way back and by the time i got home i checked it again and it was level. now this keeps happening, it seems to be when i push it a bit one of the side sits down lower and stays there until i drive it again normally. now if i remember correctly the side that sits lower is the one that did have the nut, so it makes me wonder whether that is the issue.

any ideas?