It's just odd that the two biggest ones I bought seem to be struggling the most.
That makes sense, they will have the most growth up top, but will still have been kept in a small pot, so the roots make up less of the total plant.
The small ones, will have a higher root to top half ratio, so will be quicker to take off.
You could try giving the big ones a prune back so they can put more energy into root growth.