Have you tried turning the problems into simple story problems..."if you have 3 apples and your sister has 3 apples, how many apples all together"?
My 6 year old son was doing the same thing earlier this year. 2 + 2...no idea. However, as I read more difficult story problems out loud to his older sisters- he could shout out the answers immediately. So, I started making simple word problems out of his math equations. And he could do it! We use small blocks, cars, and fingers here, too. But even when he is doing that, he will say outloud "If I have 3 apples and my sister eats 1 how many do I have left?". He does this on his own...somehow it seems to help him.
Meanwhile, we are memorizing the facts using flash cards, and he does have some facts memorized now. He takes a timed test each morning, and is getting 12-15 right in a minute. So either he is turning them into word problems very quickly, or some of the memorizing is working. And I'm ok with the memorizing, because I know he understands the concept since he can do the word problems. I still get the wild answers while he is doing his math lesson...1+4= 10? But when I get one, I just make up a story problem, he answers, and we go on. Just me reading the problems out loud doesn't help much, it is the addition of the story that does it for him. We've begun adding in more challenging word problems from various sources, and he can do so much more with these than he can do with simple equations.
Just one more thing you can try. Hope you find something that works for your girls, whether it be waiting, switcing curriculums, or something else.