Currently, if you are on certain API versions with salesforce, renewals are not recording because the interval is not being sent correctly.
It appears that the older API version(s) accept a null amount, and the newer ones do not, and require certain amounts.
"null is not a supported value for Installment Period. Select Daily, Weekly, 1st and 15th, Monthly, or Yearly." is the error message I get.
So GiveWP needs to account for any renewal intervals that are NOT in that list. Currently we send a null value for "daily" at least.