410r Rear springs issue?

[jpmatrix]

I have now owned and raced a dex410r for 4 months now and recently noticed that i have had to adjust the collar on the rear threaded shocks nearly all the way down in order to keep the ride height level with the front.

Ride height is about 18mm all round. I'm sure it wasnt like this before with the collar being half way up the threaded part when i first built it.

Checked everything to see if anything is binding but all ok. Shocks are fine and smooth. Could it be the rear springs have lost there 'springyness'?

Any ideas?