This is not diagnosis, but rather a possibility: her frontal lobe is "lazy," so the limbic center is not being sufficiently controlled for her to focus. If this happens to be the case, something like Vyvanse would "wake up" the lobe, putting it in gear to control the limbic system.
It's quite possible that your daughter has a higher IQ--probably not above average, but higher than 81--but that she could not focus while taking the "IQ test," causing the score to be lowered. Find a female Christian child psychologist in your area and let her do a workup on your daughter; if it isn't what I'm guessing here, there are other possibilities and other options.
P.S. My 17yo daughter is adopted, and my 29yo biological son was/is a certifiable genius (National GeoBee, International Baccalaureate, Rose-Hulman). Her elementary school years were plagued by the same issues as your daughter has, and she kept comparing herself to her brother, even though we never did it, and always told her she was her own person and God had His own plans for her. She is dual-enrolling in local com-col and senior year of high school, knows she wants to be a nurse, and has her education all mapped out ahead of her. I am praying a similar future for you and for your daughter.
Find a metronome app or a real metronome and teach her to use it.
Oh and there are special ways to count time by breaking each beat down. There’s no shame at all in counting each measure out this way to be sure each note is on the proper beat, practicing it slowly then getting up to the proper tempo.
I did the band thing for 6 years and was by no means talented, but I got by pretty well using this...made up for being massively uncoordinated.