As google is always prone to safety and security of mails communicated through their server. Some times google mark the legitimate mails as spam based on their filtering criteria.
When google see any suspicious thing in a mail, google will send a red banner with the mail, If you are confident about the mail then click on ‘Ignore, I trust this message’.
If you have received any mails, which is wrongly marked as spam, Select that mail in spam label, and then click on NOT SPAM option in the top.
After selecting the option, your mail will be removed from spam label and moved back to inbox.
But Still again and again, You are getting the mails in spam from a particular address, then add that address in your contact to prove the legitimacy of the user. Google will deliver all messages to inbox unless it is considered highly spam by google criteria.
You can also create a filter to restrict the going to the spam folder.