1. An org can have only up to 50,000 pending scheduled and paused flow interviews at a time in Salesforce.

SOQL to Check:

SELECT count()
FROM FlowInterview
WHERE InterviewStatus = 'Paused'

It should be less than 50000 to schedule another.

Reference Article:
https://help.salesforce.com/s/articleView?id=sf.process_limits_scheduled_processing.htm&type=5

2. Records updates may change the triggering date.

a. Go to Time-Based Workflow under Monitoring in Salesforce Setup.

b. Add some criteria and click the Search button.

c. Check the Scheduled Date.