Seems to me your suspension has jammed up; even stiff springs will give a little under your weight.
Although this is theoretically true, it can be extremely difficult to detect. A rebound spring under compression will preload a stock 120lb spring so that its initial rate is extremely high - something like 400lb/inch, as the standard rebound spring is 275 lb/inch. A 13 stone man is only 183lb, so you're not even going to shift the suspension half an inch, especially as the tyre is going to flex a bit too.
