If the user's email address was not properly sent to Blackbaud NetCommunity from the Education Edge when the NetCommunity account was created or linked, then NetCommunity will detect that it does not have an email address available for the User ID requesting the email, and will discard the email without preparing it (since a valid address is required to prepare the email).

In Blackbaud NetCommunity, Administration > Sites & settings > organization > Settings > Education, check the Preferred Email Address type under The Education Edge Options.  Ensure that the phone type for the email address on the corresponding Education Edge record uses the same phone type which is set as the Preferred Email Address in NetCommunity.  Note that these values should match exactly (e.g. "E-mail" and "Email" are two different values).

Use the "Add Users to NetCommunity" feature in Education Edge to refresh the linked account with the proper email address:
1) From the Education Edge, NetCommunity plugin screen, select the Options hyperlink to open the NetCommunity Options window. 
2) In the Netcommunity Options window, select NetCommunity Users from the Options list on the left.
3) For each set of users that you want to refresh, mark the check box and set the "Send email to type" value to the phone type which contains the preferred email address for the corresponding records.
4) Run the "Add users to NetCommunity" utility

This will cause the Education Edge to refresh the selected linked NetCommunity accounts with the email address stored in the phone type that you selected in step 3 above.  Subsequent requests from those users for forgotten password emails will now have an email address associated with the requesting User ID.