Did it start doing this after the hub was stripped and reassembled?
Obvious thing to check is indicator chain adjustment. Will it select 1st if you disconnect the chain from the shifter cable and pull it all the way out (while turning the pedals backwards)?
I've stripped, cleaned and rebuilt a couple of SA hubs. There isn't much to get wrong as long as you're systematic in taking it apart and getting it all back together in the same order. You should be able to find diagrams or photos online to help you check this. One thing that springs to mind is that I don't think the pawls are exactly symmetrical so could possibly go in the wrong way round - don't know if doing this would cause issues engaging 1st. Hardest part of the job is fitting those bloody pawl springs by the way!
If all else fails, a used hub off
eBay is normally pretty cheap and you can just swap out the internals quite easily.