Litter pan

I have two dwarf rabbits one is ten got her from the Toronto humane society
the other one is eight.I seem to be having a problem now i have two separate cages joined together for them i have noticed now they are not using there liter box always.I think it might be the oldest one.I have noticed when she is by the later box i noticed underneath there is some poop.I am really confused why this is happening all of a sudden .Any suggestions....THANKS

Two things come to mind. One is that the Rabbit(s) may now be having problems hopping into a litter tray due to the Rabbit(s) having some age related osteoarthritis. So a Vet check would be a good idea. The other possibility is if it is a new arrangement that there are two cages joined together they may both be marking their territory.