Gmail emails of known addresses automatically go to the spam folder.

For some reason particular emails i receive from known senders who's addresses are in my address book end up in the "Spam" folder. These are usually replies to emails i have sent. If i forward them back to my inbox, they eventually end up back in the spam folder. The alarming part is that the list of addresses going to spam is growing. I have several email addresses on Thunderbird, but only my gmail is affected. Any ideas how to fix this B