sounds like the struts are working ok, just not enough weight in the 2 Kayaks, at a guess i would say its set up to handle 80+ kilos,,,
resetting them may be the answer.
the was a mobile guy doing regasing in the Sydney area. google may help you, or ring a few of the local smash repairs ask who they use for regasing struts.
quick look on google comes up with a few locals,,, worth a phone call
BHG