Only 6 months? Lol...In the subject's immediate neighborhood, I'd go back 12, 18, 24, or even 36 months if I had to.
All areas from the GLA in the market grid to financing require components of bracketing and or equality and similarity.
The subject's site and GLA should be bracketed.
Location, zoning, bed and bath count, need a couple of comps with equality.
Subject's location, condition, age, views and or amenities must have similarity. Yes, I said location again. If competitive sales are this sparse, you need to find properties in alternative, competitive neighborhoods with similar median sale price points.
If you expand the parameters of time and distance to meet all of the above.... you should be good to go. You also need to explain in the report what you did and how far back you went in time and distance from the immediate neighborhood.
Remember, no one's going to find anything different than you if you do a thorough search and analysis.