add yeast at bottling?

I have made 5 or 6 buckets of cider, using apple concentrate (Black Rock NZ Cider kit). Most of it has worked out well, but twice I did not get the carbonation I was expecting in the bottles. I was never quite sure what caused the first failure. The cider tasted good, just not enough bubbles.

I have studied the process of adding sugar at bottling to achieve a desired level of carbonation. My last batch was just right. But my current batch is shaping up to be another carbonation failure. I KNOW that the sugar was correct - exactly like the last batch.

I am wondering if it would be useful to add a bit of yeast to the bucket a day or two before bottling, just in case the existing yeast had lost it's get-up-and-go.

It all depends on your methods.
ie. Are you treating with sorbate or Camden post ferm?
Are you using Dave's method of multiple rackings, cold crashings and/or gelatin to get rid of the yeast?
Are you cold conditioning for multiple months before bottling?
Any of the above, yes definitely reyeast(although with sorbate+camden adding yeast probably won't help)
Otherwise, you shouldn't need to add more, but then again it certainly won't hurt. The few times I've reyeasted, I do it at bottling time. 1/4packet to a 5G batch.

Carbonation takes time, and sometimes it's longer than you expect. I've had batches carbonate in 4 days and others that took weeks. Unless you've aged it for months there's always enough yeast to get the job done. In fact, people here have reported carbonating after a year without adding more yeast. How long have you waited, and what temperature and yeast?