Ooooo I can help you with this...
Deleate the dlc and download it with pkgj not sure what it dose different but if you look on pkgj the dlc will have a white dot next to it if its installed and blue if its downloaded
Sometime when you drop items into the vita the dont install right this was the only way I could fix the problem
(just mentioning this..................just like vita shell, you need to refresh the live area and refresh the keys database................the retropie automatically does that at random in the new update 4.4 stretch, it does sudo apt-get update, and sudo apt-get upgrade.....................vita shell may automatically run those two commands after adding content in the future) (saw that tagged on the startup command list, two restarts after a, abrupt power off I initiated)
(the problem you mentioned.......................I really experienced adding "themes" to the vita, you just dummy drop them into the "bgdl" folder, and then restart......................I once had to delete all themes and redownload them in a chain in succession, and then drop then in the "bgdl" folder for them to register, or it wouldn't work.......................you get different results from different versions of NPS Browser/pkgj, I can guess, you maybe are not using the latest build, or maybe you are?)
(those features are not implemented well, or maybe I skipped a beat, I used MTLACTIVATOR, versus, having the vita pre-activated or using that, forget what it was called, plugin to restore the activation via psn services..................I did note...................the live server connection to the official vita was hampered, after using that plugin.......................I don't, but that maybe a contributing factor)
(it was retroarch that gave me some troubles, before NPS browers developed to what it is now, troubles between version......................I updated vitashell shell then I reverted back to 1.82 i think that is the one for 3.65, for no reason, until things settle a bit with these transitions)