From the screenshots it looks like you were using bigger seats in A320 with Y70,C20 config while on A321 you are using Y174C24. Probably Leisure plus seats. You need to re-configure cabin with better seats or reduce prices.
90 seats in a A320 vs 198 seats in a A321, and I guess you don’t have a lot of space left in the A320. So it may be the same service, but not the same seats. And yes, that can definitly make such a huge difference.