.img has to be in sd in a Garmin folder (G capital letter) .img must be named gmapsupp.img (check hasn't been renamed by mistake gmapsupp.img.img ,enable extension in your computer and check)
no .unl file or .gma file must be present in sd or in astro internal memory concerning this map (it's unlocked no .unl and .gma needed or it wont work).
If the same map in the same sd works in 62, it's your Astro problem.
Check also you don't exceed maximum maps tiles in astro with javawa device manager h ttp://w ww.javawa.nl/jdm_en.html
install it and see what it says about your astro tiles.
Again if you have other maps covering the same map area deselect them.
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If Gb map is in astro Maps list, it's correctly unlocked and astro sees it, if it doesn't show it's not the unlocking problem, but another problem.
If you have another GB discoverer partial map in astro remove it, astro wont show more than one file with the same FID.