Kirsten,
A couple of things to check at this point would be to see whether you are getting to the activities scheduled within the Bigger Hearts guide for 2A/2B. This makes a big difference on how well kiddos understand what they're being asked to do.
Another thing to check is whether you are following the schedule within the Bigger Hearts guide for the amount of work to be completed each day. I mention this because pacing also has a lot to do with how well kiddos retain.
Next, I'm wondering if you are staying with a concept longer if she doesn't seem to be getting it, or if you are moving on? Typically, we would advise sitting with her and helping her as much as needed, problem by problem so that she is successful with the math. We would also advise not lingering on a concept longer than it is presented and just moving on the next day, knowing it will come around again when she is more mature. Maturity does a lot for helping kiddos understand why a math concept works.
Often just being present during the math lesson, teaching the hands-on, doing the lessons at the bite-sized pacing suggested in the guide, moving on to the next lesson even if she doesn't seem to have fully mastered the previous one, and partnering with her to figure out each problem fixes many math woes. It's good to remember that she is not supposed to master all of the concepts in 2A and that we have found that the beginning of the A book of each level is quite a jump. Singapore has a great ebb and flow, so if you move on each day, you'll actually move away from areas that may have been frustrating into areas that are less frustrating. Then, when the concept that was so frustrating comes up again, she'll be older and more ready to handle it. If your oldest son was a mathy kid, all of these things I mentioned above may not have been needed for him, but with your daughter they will make all of the difference in the world.
I wouldn't worry about any special testing at this point. Some kiddos are just more mathy than others. Lots of patience and sitting next to her and talking through each problem will help the most. I did it for years in the public school classroom with all sorts of kiddos across the math spectrum (who were all using the same math curriculum) and my constant presence during math time, my patience, and my guided help with talking through each problem made the biggest difference.
Blessings,
Carrie
And it's not just one subject - it's all of it - adding or subtracting, money, time - somedays it's clear as a bell and days like today it's like I never taught her anything. I have let her dad teach her just to see if it's me and he encounters the same thing. We both feel like she is not trying. But how do you know if there is a learning problem? She is reading OK - finishing the emerging readers, and can narrate fine. 
So, your sweet daughter will eventually get this! You are not alone in your frustrations. Money and time are hang-ups for many kiddos in the introduction years.