web-2-lead form validating and returning to right url but not adding new lead

web-2-lead form validating and returning to right url but not adding new lead

Hi,

We've integrated the code from the web-2-lead form. When entering data en submitting, the form leads us to the thank you page. However,  no new lead is generated in CRM. 

What are possible causes?

Thank you,

Jochen