First, understand how a diverter is meant to behave, because a lot of "faults" are actually normal operation. An Eddi's main job is to use surplus solar to heat water, so on a dull day with little generation it won't have much to divert and the water won't get hot from solar alone — that's expected, not a fault. If you want it to heat from the grid regardless, you have to explicitly force a boost; left to itself it only acts on surplus.
A few common real issues and checks: confirm the immersion thermostat is working and set to a sensible maximum — if the diverter thinks the water is already at temperature it won't kick in, and a faulty thermostat can cause odd behaviour even after you've replaced it. Check the firmware version in the app (Settings → Device settings), as outdated firmware causes glitches. If a forced boost only ever pulls a trickle (e.g. 0.1 kW) instead of full immersion power, that points to a wiring, thermostat or element problem worth raising with your installer.
Also remember the diverter can be put into a "stop" mode (handy when you're away) which prevents it heating on both surplus and timer — worth checking you haven't left it there. And note that if the water's already hot from another source in the morning, the diverter correctly stays off until the temperature drops below the thermostat setpoint. Distinguish "behaving correctly given low solar" from "actually faulty" before assuming the unit is broken.