A Web-to-Lead form creates a lead without a person in the room, so nothing notices when it lands in the wrong place. These are the causes worth checking, in order.
1. No active rule, or the wrong one
Salesforce allows one active lead assignment rule at a time. If someone activated a different rule, or none is active, leads go to the default lead owner you set under Lead Settings.
2. The form sends values the rule does not expect
An entry that tests Country for United Kingdom never matches a form that posts UK or GB. The same goes for state, industry and any picklist: the form must send exactly what the rule tests.
3. The lead arrives with the wrong source or missing fields
Rules often test Lead Source or a campaign field. A form that does not set them skips every entry that depends on them and falls to the catch-all.
4. A flow runs after the rule
A record-triggered flow on lead creation can change the owner after the assignment rule has run, and has the last word.
5. The form has a daily limit
Web-to-Lead allows only so many leads in a day, and the figure depends on your edition. Check the current limit in Salesforce Help. Past it, leads are not created, so they are not misrouted: they are missing.
Check it yourself
SELECT COUNT() FROM Lead WHERE IsConverted = false AND LeadSource = 'Web' AND Owner.Type = 'Queue'
SELECT COUNT() FROM Lead WHERE IsConverted = false AND LeadSource = 'Web' AND Owner.IsActive = falseUse the source value your form actually sets. Then submit a test lead from the live form and see who owns it.
Have it checked every night instead.
Routing Forensics connects as a read-only user and checks this, and the rest of your routing, against your real leads. Register interest and you will be first to get a free health report: what is leaking, where, and what it is worth.
The free scan cannot see this one: it reads assignment rules, flows and queues only. Connected, it is checked every night.