Hi Becky,
This is a good question that comes up in the HTF occasionally. I can think of several ways you might deal with a soft spot, although J & R Repair probably encounters this problem periodically and may have developed quick and cost effective spot repair. You should contact them first.
Generally water is the root cause of soft spots, so even before repairing the spot try to find and stop the source of water seepage/leakage. What caused you to notice the soft spots, is there some water staining or other discoloration? Are there depressions where the soft spots are located? Any old picture nail holes on the inside wall? If there is or was a towel rack or hooks in the vicinity, are the holes or mounting flanges sealed? Etc.
Good luck.
Jim