By default, Gmail has conversation view turned on which means all emails with the same subject title will collapse into a single line item . For many people this is really confusing and looks like emails are missing because they actually be farther down in the Inbox collapsed with an older mail. To turn this setting OFF, do the following:
• Click on the gear icon at the top right of your Gmail account.
• Scroll all the way to the bottom and be sure Conversation View is UN-checked.