New reviews haven’t appeared yet
The short answer
New reviews appear in the widget on their own, but not instantly. If one has not shown up after a few days, the cause is usually a filter, a sorting setting, or the source having changed.
There is no manual refresh, and support can adjust the interval in some cases.
What to check, in order
Sorting is set to most recent first — with another sort a new review can be sitting well down the list. No filter is excluding it, by minimum rating, text-only, photos-only or keyword. The source connected to the widget is still the right one and has not been recreated on the platform.
And the review is actually live on the platform. Reviews are sometimes removed by the platform within minutes of posting, in which case it never existed as far as the widget is concerned.
The 100-review window
The widget holds the 100 most recent reviews per source. On a busy profile a new review pushes out the oldest, which occasionally surprises people who were watching for a specific older one.
If it is still missing
Send support a direct link to the page with the widget, the installation code, and a link to the review source. Custom adjustments to the refresh interval are possible in some cases and support will tell you whether yours is one.