The Challenges in Fishing Clash are usually related to the ongoing Event, which also determines their duration and how many become available during the day. To ensure a Challenge stays interesting and competitive, anglers are matched in "Challenge windows" that group them together for a specific time.
When a few anglers enter the game, the ongoing Challenge becomes available to them to clash in; a new "Challenge window" is therefore created. Any players who enter the game at a similar time become matched with those anglers, which may explain why they may have a few minutes less to participate in the Challenge.
In some cases, when there's not enough time to compete with anyone in the ongoing Challenge, anglers may not have it available. Not to worry! The next one will start at the time indicated on the main screen. Pro-tip: it's worth checking the in-game inbox for the specific Challenge schedule to avoid missing out on any of them. Anglers may access it by selecting their avatar from the main screen and clicking the inbox tab.