Are you having trouble fingering the chords they're asking for correctly? The only way to get better at that, I'm afraid, is to practice fingering those chords. Take the first two finger positions of the song you're trying to learn, and practice shifting between those two chords. After each shift, you should strike the strings to make sure each one is ringing clearly. Correct any mistakes. Then shift to the other chord (even if it's easy). Keep practicing that transition.
As your fingers become more used to that chord and how to get there quickly (when I first started, I had to fret each finger one at a time to make a chord, open C maj to open G maj would always cause my third finger to lag behind the others in finding the right fret. Took a couple months to even be able to shift between those two properly and at speed, but now I can play GCDC chords, or any other similarly easy open-position chords (Am, D, Dm, D7, E, E7, Em, A, A7, G, G7, etc.) with relative ease in quarter notes) you'll notice it gets easier to quickly shift between the two. Once you can move fast enough, try to finger the next note. You should only work on transitions that you can't perform quickly, if you come across a new fingering that you can get to relatively quickly, you need not practice it as much right away.
Another tip is to practice the Pentatonic scale in the key of A (google it), or the chromatic scale. In fact, you should practice the chromatic scale to warm up and cool down every practice or play session. Look 'em up on google.
Basically: it just takes practice. Your fingers aren't functionally unable, you've just never asked them to do these things before. So at first it's going to be awkward and sound pretty terrible. Think about the awful drawings and writing assignments you got A's on in elementary school that your parents hung on the fridge anyway. But after more practice, you'll start to get better and notice improvement.
