It could be the pot and fast leaching of nutrients such as calcium & nitrogen .. both are essential for growth. Also the type of potting soil matter. When I grew roses in MiracleGro Moisture Control potting soil .. roses grew fast & tall due to plenty of fertilizer already there, but I switched to cheap Schultz potting soil and their growth got stunt. MG-moisture control retains water & nutrients well. Meillands like Charles d. Gaulle are bigger waterhogs than Austin roses so a potting soil high in peatmoss helps.

Last year using alfalfa pellets plus ORGANIC soluble at NPK 4-3-2 WAS NOT ENOUGH. A dozen roses were wimpy & stingy in pots. But previous years using chemical Soluble for roses NPK 19-18-20 they grew twice bigger & more blooms. Pots need higher NPK, more frequent soluble fertilizer since nutrients leach out fast. Pots don't have enough soil bacteria to fix nitrogen, so chemical fertilizer or SOLUBLE fish emulsion is best.

Years ago when I first grew Austin roses, I made the mistake of giving them high nitrogen NPK 10-5-4, and they got too tall, little blooms. Now I give all my roses low-nitrogen & high potassium, and they are shorter with more blooms. But I still want the tallest-possible roses since the rabbits chew on the short ones in the spring. Question for everyone: What are your most vigorous and tallest roses in spring? Thank you.