By default, when trying to match visitors cookies are not available, Matomo looks back only in the past 30 minutes (more details). We do this for performance reasons: to keep Matomo tracking fast, even if you track hundreds of thousands or millions of unique visitors per month.
For example, to configure Matomo to lookup a visitor up to 6 hours in the past, you would need to set the setting to 6*3600 = 21600 seconds under the
[Tracker] section of your
[Tracker] window_look_back_for_visitor = 21600
Note that increasing the value of this setting may have negative performance impacts on your Tracking API requests speed.