The emails are most likely being delivered to the specific email provider, however, the emails are being placed into their Spam folders, rather than their inbox. This is based upon the level of user interaction (i.e. - users opening and clicking on the messages).

Gmail (and other email providers) determine if emails gets 'bulked' or 'inboxed' based on user engagement. If users don't engage in your emails, then they determine that it is unwanted through user engagement metrics like past  opens and clickthroughs. For Gmail, they also put a lot of weight on users adding the sender's address into their own whitelist, which tells to Gmail that they do want the mails going forward.

Moving messages from the Spam folder to the Inbox will require the recipient users to either move these messages into their Inbox and/or add the messages from the address to their personal whitelist. It is a best practice for clients to include a snippet/statement in their email messages to remind users to whitelist the messages, which ensures they have full visibility of future messages from the organization.

Unfortunately, when it comes to a content-based, mailbox placement predicament, we do not have any authority nor remediation for such events especially when it relates to Gmail. The best way to avoid emails being sent to the spam folder is to use Engagement Factors and target constituents who are more active. Additionally, make sure that you aren't using spam phrases, do not use any CSS and maintain a healthy balance between code, text and image ratios.

Here are a few other suggestions by Google on how to avoid Gmail spam filtering:
  1. Tell subscribers to watch for your email messages. Emails that confirm a purchase or new subscription are among the most opened emails. Use these emails to tell your subscribers to watch for upcoming emails and to check their spam folders if they don’t receive them. You may also want to remind them to let Google know that your emails are wanted by clicking the “not spam” button if they do find your emails in their Gmail junk folders.
  2. Ask subscribers to add your “from” address to their Gmail list of contacts. This will help ensure that your emails don’t get filtered into recipients’ Gmail junk folders.
  3. Google recommends not using pre-checked opt-in check boxes. Make sure that it is obvious to the user that they are opting to receive email from you.