It may be one of the following reasons.
1. Check if your open tracking enabled for your campaign
Open tracking must be enabled before the campaign is sent to track opens in the campaign report.
If open tracking was disabled for a campaign, there you won't able to see any count.
Please read how to check and enable the campaign's open tracking.
2. Wait for enough time to reflect on the open count
It's common to open tracking count will take some time to reflect the count on the platform, after opening any email.
Although it is working in real-time and will show you the count as soon as some open your email, still it may take some depending upon the system scheduled job to be run and update this count for you (for a few minutes/second).
3. Is your email campaign a plain-text campaign
Open tracking depends on this tiny image pixel hence it only works in the HTML email and not in the Plain-Text campaigns, since you can not include the images in the Plain-text email campaigns.
4. The tracking image may be blocked
Open tracking won't work if your contacts or their email client have chosen not to display images.
Some email inbox's spam filters may block our tracking image too.
In a few email clients, the images are by default disabled hence system won't able to track it.
E.g. In the Outlook Email Client
Your contact can enable these image visibility options.
Although your click tracking will be still working even if the open tracking won't work.
5. Gmail has clipped your message.
If an email message is too large, Gmail will clip content and hide it behind a View entire message link.
When the message is clipped, it also clips the tracking code that accounts for opens.
To prevent this, reduce the message size of your email.