eM Client not downloading all messages

A few times a day there are emails that don’t show up in eM Client 7.1 that are present in our web mail account.  Are there any filters or other reasons why they would not be showing up in eM Client?  They are not going into the Junk folder.

They may be included as part of a conversation, and are therefore not obvious.

Can you find them in eM Client if you search for them, or if you disable conversations? (Menu > View > Conversations)