I think you two just picked the wrong contraction! I personally think it is simpler not to have any contractions and I have never been "wronged" for not using one. But this iteration Duo is throwing around - and also accepting - stuff like don't. My advice is only use a contraction you've seen Duo use! Tough luck...
I can't give you a thorough explanation, but one simple guideline that I learned is that "aquí" is here and "acá" is more like "over here". It's actually a little clearer with "allí" and "allá"--"there" and "over there". I know that I've heard "Ven acá" ("come over here") frequently.
I actually had a chance to use "allá" in a Walmart, when I asked an employee who didn't speak much English where to find something, and she gestured over somewhere that looked rather distant, and I asked "¿Allá?" and felt very pleased with myself!
I've also read that acá should be used with verbs indicating movement towards the speaker, while aquí should be use to describe the state of things or locations of physical things (estar). I'm going with this usage because it appeals to my logical self, so is easier for me to remember, regardless of what local regional variations there are in usage.