Checking the current Factorio wiki, the mk2 power armor appears to require 30 low density structures in its recipe, though they only have a stack size of 10. This would be a "dense recipe" issue, where if you were allowed to recycle it, you would lose the 20 extra low density structures, due to limitations of the Factorio engine (at least as far as v0.16 was concerned)
If you still want to recycle this type of item (alongside others, detailed in the second FAQ available here, then you will have to disable the safety toggle setting. Then you should be able to recycle it.
Another alternative is increasing the stack size of the low density structure, using another mod, such as ReStack, Jamozeds Tweaks, or Noxys Stack Size Multiplier. (Some of those mods might not have low density structures as an option, I haven't personally checked)
I'll add the Mk2 power armor to the list of dense recipes in said FAQ, so thanks for bringing it to my attention.
