Why aren't my emails fully downloading on my iPhone?

Hi, I got the new IPhone 16 pro last week and transferred existing phone to new phone and updated with the IOS 18, all appeared to go well, except have noticed whilst all my emails transferred to new phone a lot of them haven’t downloaded properly ie can see email heading but not the actual message and or any email with attachment ie PDF just won’t download when I click on it it’s like a picture if that makes sense. IPhone storage is fine so not sure what’s happening or what’s gone wrong or how to fix it, can anyone help please? Thanks