The Adeptus Terra also has a strong smurfs bias*, this may actually play into some chapters difficulty of rebuilding. Per Codex, more than half of current chapters are direct or indirectly founded from Ultramarine geneseed.
I think most cases of persistently below strength chapters are a function of either bad luck, too aggressive, having too harsh a training program, or having a hard time recruiting sufficient numbers. I don't tend to think of it as a gene-seed issue. We also see some BL novels where traitors target gene-seed repositories for their own ends. The traitors often are trying to get their hands on geneseed, that also plays into the idea that loyalists may actually lean on other sources than harvesting "heroic" gene seed from brothers.
*EDIT- notice the smurfs didn't seem to lament too long about rebuilding after the nids invaded.