The Renix fires the injectors separately on the plus side and share a common ground. Best guess would be an iffy CPS, do Cruisers CPS test and also do a CPS ohm test. The CPS can be temperature sensitive and work well when cold but screw up when hot.
The injectors ground goes through numerous splices/connectors and eventually ends up at the dipstick holder. An ohm test while shaking the harness might tell you something.
There are other reasons, but these are a good pace to start and the more likely causes.
If you are testing them while cranking, the CPS won't trigger the injectors unless the motor is turning over well (some say around 300 RPM). If the motor turns over too slowly it can cause issues with the CPS/ECU and functions like the injectors firing gets iffy.