Why doesn't my pear cordon grow?

I have a row of 6 cordons planted 3 years ago. The second one along is a Packhams Triumph. It leafs up in spring and flowers well but grows no new stems or leaves, the trunk hasn't thickened, it sets no fruit. I treat it the same as all the others and they grow like crazy and fruit well for their age. I've dug around the roots and they look OK. I've sprayed it with seaweed and fed it extra, but it's stuck in a Groundhog Day, does the same thing every year. Any ideas? Will go on the rubbish heap this year if no-one can help it but I'm concerned about having the same problem with a replacement. The cordons are only 3 foot apart.