Not sure what you have been reading but there is nothing fast about a climbing hydrangea :-) And because Miranda is variegated, it is less likely to grow even as "rapidly" as the unvariegated form. Establishment of these climbers is generally listed as 5-7 years before you see significant growth and heavy flowering. Larger plants to begin with often take a little less time but patience with any of these is a requirement.

Heavy shade may be a bit intense and could be slowing growth. Some sun is ideal to develop flowering as it should, so like its shrubby cousins, I usually suggest morning sun and afternoon shade.