Not always. Our school psychologist caught my daughter's processing problems.
Strangely (I assume this is strange), at almost sixteen, her last test did not show any processing problems, but she still exhibits problems understanding verbal input and has a rotten memory...both verbal and auditory. She does get accommodations for these issues and her reading, which was VERY affected by her auditory and visual processing disorders, has caught up to age level. but OH that math!!!!!! My own way of doing things is to get the dx. you need to get the help and then get the help. I really don't care what they call it, as long as the school is willing to help. They could call it "I Have No Idea" disorder, and, if the symptoms were accomodated and worked on in school every year, that would be FINE and DANDY with me!
Whatever my daughter has, and no two professionals agree, she got help throughout her school years and her grades have been B's and C's rather than F's and she is much more able to do school work independently than she used to be. So her diagnosis called, honestly, "WE DON'T KNOW" was accomodated and helped A LOT and that is the bottom line for us. She is one of those kids who obviously has learning differences, but nobody knows exactly why or their cause. If you have a "WE HAVE NO IDEA" kid, just get a diagnosis and hope the school steps in with a lot of interventions to help with the issues. Amazing things can happen when the educational people really step up to help kids who need to discover alternative ways of learning.
On the other hand, if the child doesn't get some diagnosis that will get him/her help, a lot of bad stuff can happen to the child's education, such as he/she can be labeled "lazy" or "a problem" or "bad" or just allowed to fall through the cracks. That was never going to happen to either of my kids. I went through that myself and it's terrible.