I took a look at the recipe again. Don't know how I missed it but the yeast is an English ale yeast. English strains drop out fast as the temperature of the beer begins decreasing as fermentation activity declines. I once bottled a beer that remained at 1.018 for two weeks. Four or five weeks later I noticed the carbonation increasing. Disturbing some of the yeast during bottling and the warm conditioning temperature got the yeast active again. Had the same problem with a porter that dropped 7 gravity points after bottling.
Try rousing the yeast with a long sanitized spoon and warm the fermentor a degree or two over what it has been at for the last week. This may reactivate the yeast that remains in the fermentor.
I now maintain the highest temperature reached during active fermentation until I'm ready to bottle. Bottling is not less than three weeks from the beginning of fermentation activity.