Did the Marcgravia have much of a root system? I think that many times these things get sold as recent divisions. Plants have a give and take relationship between hormones produced at the roots and at shoots, auxins and cytokinins.
For example, without actively growing roots cytokinins aren’t present in great enough amounts to warrant rapid shoot growth. The opposite is true as well.
Hopefully as your Marcgravia become better established they will begin to grow rapidly.