By default, when trying to match visitors using a fingerprint (for example when 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 30 days (30*86400 seconds) in the past, you would set:
[Tracker] window_look_back_for_visitor = 2592000
Note that increasing the value of this setting may have negative performance impacts on your Tracking API requests speed.