Baby not gaining enough

I have 6 month old twins that were born 7 weeks premature. They’re EBF and have been gaining great until this last appointment. They were both gaining about 6 oz a week up till there 4 month appointment but now at their 6 month appointment Baby A gained 4-5 oz a week and Baby B only gained 3 oz a week. Their doctor is worried about Baby B’s growth since she wants her gaining at least 4 oz a week. She is meeting milestones and has plenty of wet and poopy diapers but idk how to get her to gain more weight. She is very content and eats on demand about the same amount as sister usually 8-9 time times in a 24 hour period 5-6 during the day 3 at night. What can I do? I don’t want to supplement with formula.