According to the RHS, propagation is achieved by basal softwood cuttings, or by division in spring - have you tried dividing it?

plant not large enough to divide. have tried softwood cuttings in several different mixes with no luck.

Well, here's the RHS information on how to do softwood cuttings - in this case, the cuttings need to be from the basal shoots, and you'll need to wait till next spring. Use starter potting soil (we call that seed and cutting potting compost, note the difference in terminology - potting compost is equivalent to your potting soil).

https://www.rhs.org.uk/advice/profile?pid=307

I've grown it from seed, never thought of taking cuttings!