The Lead conversion to a contact will fail if you are not filling in the entire address for the lead. There won’t be any message telling the user that the address is insufficient just a failure. The reason for this failure is every contact must have an entry in the address table so the InforCRM client is looking for that data to fill in the address table. If you provide the Street, City, and Zip you will be able to convert that lead to a contact.
What else do you need to convert a lead? You must have a valid leadsource, and usually, that will be filled in but if you have moved from a LAN based system and did not have leadsources in the leads table then you could be missing a leadsource. Please check to make sure each lead has a leadsource.