The only one problem I have had on my Citroen Picasso is with the tailgate, which does not always lift fully and has to be pushed up. I brought it to the attention of the dealer, but was told the struts were within Citroen's specifications. Subsequently when I again reported it I was told the struts were a wearing part and not covered by the warranty. That implies that I should cough up for new struts every 12,000 km or so. Citroen referred me back to the dealer and the dealer states that they are hamstrung by Citroen policy. Meanwhile the warranty period is about to expire, and I am ignored as a disgruntled owner.
It would appear the gas is leaking from the struts, and quite rapidly it would seem given that they are in effect running out of gas in 12,000 km. Go back to the dealer and Citroen and try to negotiate a new set of struts under the warranty, or alternatively have the struts regassed independently. I suggest the latter would probably fix your problem.