Welcome HayJo,
I can't help you with understanding the MAPS scores, but fwiw as I've watched each of my kids and their friends go through preschool and the first years of school, reading seems to be an ability that rather than moving upward in a linear trends grows in spurts... so I wouldn't necessarily worry about it unless you have other reasons to question if there's an LD or other issue.
My youngest dd is in 2nd grade this year - she's a kid that everyone (parents, friends, teachers, etc) think is at least HG based on her ability to pick up new concepts quickly and her verbal communication. She started learning to read in preschool which wasn't a surprise, but her growth in reading stagnated by the time she reached first grade and by the end of first grade she was very stressed out by reading. She's just been through an eval and was found to have a challenge that impacts her ability to read. So - fwiw - the non-progress in reading achievement scores *might* mean there's some kind of challenge - if yr ds has shown other signs of challenges such as not wanting to read out loud, stumbling over words, seeming to not recognize words you think he should know when reading, etc and your gut feeling is telling you something's up, I'd look into it further.
Best wishes,
polarbear
