Some emails downloaded to KMail 1.9.9 just "disappear"

I have been having problems with POP 3 and Gmail in that some emails are there but seem not to download. I have established (with the expenditure of some time) that the mails download (I have sat and watched them do so) and then disappear. I have forwarded the mails through two Gmail addresses and one totally different email provider. In every case they simply disappear. They don't even land in the wastebin. Has anyone any ideas, *please*. My next idea is to destroy all my filters and gradually restore them and see if the recalcitrant emails are being misfiled in some mythical and non-existent folder. But that is a large task, and I am loathe to do it if someone can offer something easier or more sure of success. I would be doing it in desperation, and not in expectation of success. Thanks, Lisi