Hi Hilly
As there is no adjustment on the Exo's there is only a couple of reasons for this
First check all the bearings for movement and the bracket under the foot to see that it is tight on the frame also check all bolts
If all of these are reasonably tight you have propably got the spring missaligned
The spring is hald in place by 4 bolts two through the spring center and two at the top
If these are loose the spring can move to one side or the other
The simple answer is to loosen the 4 bolts, pull the spring into line and then tighten them
Post the pics if you can as I might be wrong
Jason