If you tried all the possible steps in WordPress forums, like fixing the image size based on installed theme, and your site logo still won’t appear, you could try this step to fix the problem.
Step 1: Go to your Dashboard, click Appearance, and select Customize
Step 2: Click Header
Step 3: Click Site Identity
Step 4: Uncheck the Retina Devices and Mobile Devices options
Step 5: Go back to Header settings, and click Transparent Header
Step 6: Uncheck Transparent Header option
Step 7: Publish and check all the pages if the logo is now appearing.