Have you had a good look at the output/log files?
There may be a reason why the measure is not computed. My knowledge of mechanisms is not so good, but I know from other software that if you do not constrain your mechanism correctly, sometimes the software adds or releases constraints. If your measure is on one of these locations, it may not be calculated.
In this case, the system may be 'overconstrained' due to using 2 cylinders. The software does not know how to distribute the load between the 2 cylinders. It may help to suppress one of them and then rerun the analysis. But this is just a wild guess, hopefully your logfiles will give some clues for a more educated diagnosis.
Offtopic: in reality the cylinders may also give a different force, and you may be misaligning your assembly. Something to think about maybe.