Help troubleshooting heater failure?

[rennjidk]

I have 2 75w neotherms (150w) in opposite chambers of a nuvo 30l, with the return pump in between them. My tank (27g) is struggling to maintain temp at room temps below 70F. They are both set to 78 and connected to an inkbird, but the tank never goes above 77F (confirmed with 2 other thermometers) and occasionally drops to 76F. The only thing I can think of is that one of the heaters is intermittently failing but I can think of how to determine which. Disconnecting either one of them will cause the tank temp to drop regardless of whether the connected heater works or not.

Edit: they both are displaying lights indicating they are heating and my ambient room temps are 70F (day) and 66F (night)

 

[Cell]

Sounds to me like you need a bigger heater. Preferably a brand not known to explode.