An invoice showing fewer hours than you expect is usually correct. Work through these.
Those hours were already billed and paid
The most common reason. Every paid invoice records exactly which days and projects it covered, and a new invoice subtracts them.
The subtraction is by the hour, not the day: if four hours of a six-hour day were already paid, the new invoice bills the remaining two. Only paid invoices hold hours back, because drafts, issued and cancelled ones reserve nothing, so re-issuing is never obstructed.
Where hours were withheld, the invoice tells you which dates and why.
A very old paid invoice blocks its whole period
Invoices created before per-day coverage was recorded do not know exactly which hours they covered, so they block their entire period instead of risking billing something twice. Safe, and occasionally more conservative than you want. If you need those hours billed, cancel the old invoice or raise a custom one.
Some hours could not be priced
Unpriced hours do not silently vanish. They appear as a line with no rate and hold the invoice in draft. If the invoice is in draft, that is your answer.
The period is not what you think
Invoices cover a pay period measured in your organization's timezone. Work at the edges of the period can fall the other side of a boundary. Check the invoice's own dates instead of assuming a calendar month.
The project or member is excluded from automation
Check the Automation tab. An exclusion set up months ago and forgotten is a classic cause of a client who has not been billed since spring.
The hours are on a different project
Time tracked against the wrong project appears on that project's invoice. The timesheet is where to confirm and correct it, and the invoice re-prices afterwards if it is not yet paid.
Another live invoice covers the same days
Two unpaid invoices covering overlapping dates means the same work is about to be billed twice. StaffVertex warns you about this on the invoice and after a payment. Cancel the duplicate instead of editing both.
Still short
Compare the invoice period against the Time & Activity report for the same dates and project, with past projects included. That comparison narrows it to one of the causes above almost every time.