Users have noticed a change in functionality with custom forms. They can no longer create a record on a lookup with just one click; it now takes several clicks to get there.

Why can I no longer create a new account or contact from the account/contact lookup when creating a participant?

This is a platform limitation with lightning forms (and standard forms do not have this issue).  To use this functionality again, follow the steps below:

Step 1 - Input at least 3 to 4 letters of the record they are looking for and then click on the magnifying glass. 

Step 2 - Once the new window pops up, click inside the lookup box again and there is the '+ New Account' option.