Some tattoos downloaded from the gallery don't get applied to Sim

I do not think the tattoos you save are supposed to appear in CAS (alongside the defaults in the individual menus, if that is what you mean). They can be applied directly from your gallery or the public gallery. They still will appear on the sim, but the indication that a custom tattoo has been applied is that the icon to open paint mode will be rainbow colored instead of grey. But the tattoos you save do not appear in the CAS menus the way that custom content might, by design. 

If I have misunderstood your issue, I apologise, but I hope this helps.