Do you know if this particular fish had all barbels present when you acquired it? It may have come from an injury at some point, or even had it missing since it was young.
One of my larger clown loaches (who used to be the Alpha loach until Marge came along) has not got much in the way of barbels on the lower lip. This I put down to her being kept on a gravel substrate for some years before we acquired her, hence barbel erosion. I'll try to dig out a picture.
If it is a recent injury, keeping the water in tip-top condition may aid its regeneration. Keep a close eye out for infection though.
Here's a pic of the loach I mentioned. She's over 14 years old and about 9" TL. Since rehoming her into our group (some 5 or 6 years ago now) the barbels on the lower lip never regenerated:
She also has a funny tail fin too, but I love her just the same.