xliioper

Z fares have U/T/X/V fare basis codes (and thus require at least U to be open to book), while I fares have Q/K/L (although cert allows either Z or I fares so having I and Q open should be sufficient for cert as long as any fare rule requirements are met). Not clear if poster was trying to use certs for coach or FC. Coach requires L/U/T/X/V booking availability and it may never open for popular flight times.
In addition to EF, you can set Google Flights price alerts (can't set for a specific fare bucket, but if you are familiar with fare filings on route you can see if price point fall to point which indicates availability in fare bucket).