The usual causes, most common first: Google's spam filter caught them (reviews from brand-new accounts, bursts of reviews in a short window, or reviews left on the business's own wifi/IP all trip it), the reviewer's account got flagged, or your profile has a temporary quality issue that pauses review display.
The burst pattern matters most for legitimate businesses: if you ask twenty happy customers at once, a wave of same-day five-stars looks manufactured to the filter. Steady asks, every job, every week, produce reviews that stick.
You can't appeal a filtered review directly. What you can do: keep the ask cadence steady, never review from your own location or devices, and make sure your profile itself is clean. Filtered reviews sometimes reappear once patterns normalize.