Answer: How perplexing! It is possible that for some reason the two have not rooted properly, or possibly their grafts are not perfect. It is also possible that unbeknownst to you there is an underground, invisible difference in their growing conditions such as bedrock interfering with their growth. Without digging them up to investigate to learn the cause, it's a bit difficult to know what to suggest. You mgiht wish to contact a professional arborist for an on-site opinion, or your County Extension might have some ideas. Their telephone number in Binghamton is 772-8953.
I hope you solve the mystery!
Q&A Library Searching Tips