It sounds like you're doing all the right things. There's one more thing I can think of that I haven't seen mentioned here. You could try hanging up some shelves so the cats have the ability to get up into a high safe place with an escape route.
Have you ever seen the TV show called "My Cat...
Hi AmyRose.
Did you talk to the local no-kill shelters in your area? Do a google search for them. They would be the best people to give you advice as to what your options would be and they can provide a trap so you can catch her. Keep in mind that, even though they are no-kill, it's been my...
