If anything, your charging rate is too low. One tenth C is about right for a single string. That works out to 215AH x 0.1C = 21.5A. Since you have two strings, it's 2 strings x 21.5A = 43A. To be clear though, the 40V is the raw solar voltage going IN TO the controller? What is the charging voltage coming OUT OF the controller? It should be about 13.5-14.4V, depending on the charge of the battery. That's how a MPPT controller works. My gut feeling is that you've been undercharging for a while now. You're supposed to hear bubbling in a properly charging battery. Don't add additional batteries unless you add more panels at the same time.
