Centranthus ruber

Bought and planted two young Centrathus ruber (or rubra?) plants in May.  So far, they are very thin-stemmed, don't seem to fill out at all and the flowers appear for a few days and die off.  This may be what they are supposed to be like as young plants, but I had expected a bit of a change or show of strength by now.

Can anyone tell me if this is just what they will naturally be like in the first year, being so young?  I haven't supported them and perhaps should have to stop the stems bending into non-upright shapes.

Any advice would be grateful.  I had read they do well in dry soil (I have clay with manure dug in) so perhaps they hate my soil and are rebelling!?