Work down this list. One of these is usually the answer.
The hours are on the day before or after
Days are grouped by your organization's timezone, not your computer's. Late-night or early-morning work often lands on the neighbouring day. Check the day either side before assuming anything is missing.
The desktop app has not synced yet
Time is written to your machine first and uploaded afterwards. If the app was offline, or was closed before it caught up, the hours exist locally but have not arrived. Open the app, let it connect, and give it a minute.
Idle time was discarded
If your organization discards idle time, a stretch where you did not touch the keyboard or mouse is removed from the entry. Long reading, a phone call or a meeting away from your desk can all disappear this way. Your organization decides the threshold and the behaviour.
The day exceeded the maximum and was flagged
An entry beyond the organization's daily ceiling is held out of totals until someone reviews it. It shows on the timesheet marked as exceeding the limit. This is usually a timer left running overnight. Correct it and the hours return.
You were tracking against the wrong project
The total for the day is right but a project looks short. Check the entries themselves instead of the summary, and use edit to move them.
Two entries overlapped
Overlapping minutes are counted once. If you recorded the same period twice, for instance a manual entry over the top of tracked time, the total will be lower than the two durations added together. The smaller figure is the right one.
You are looking at the wrong organization
If you belong to more than one, check the switcher at the top of the page. Hours tracked in one organization never appear in another.
Still not right
Note the date, the project and the hours you expected, and send that to your administrator. They can see the underlying entries, including ones flagged for review that do not appear in your totals.