Mail app not syncing with Gmail online.

Hi. I have an iPhone 11PM 16.2os. Recently, when I read my mail on my phone, reply, delete etc. I come back later to my mail online, Gmail. And all those new messages are still there, as if unread. The replies are in the sent folder. Weird. I checked my imap settings, on. I set data to be fetched, not something I had undone. In fact , I haven't touched the settings on my phone since I can remember. Does anyone have any ideas? I'm short of deleting and setting up the app to see if that works.

Tia,

Lindsay