I'm not sure what you mean by Long Tom pot, but Romneya is a pretty large shrub, wanting a good bit of root room, so if your pot is less than a foot deep and wide, its not big enough. You'll need a large container, minimum 2 feet deep by 18 inches wide, preferably bigger, or to put it in the ground. The other point is pruning - try cutting it back late winter rather than in Spring to give it more time, in hopes it might actually flower on its new growth, but it does seem as if you're already pruning correctly anyway. Only other thing to say is that this plant prefers full sun and well drained soil, so overwatering or overfeeding, particularly with a high nitrogen feed, won't help.

Many thanks - much appreciated; I think I better put it in the ground!