Alerts - unreliable! Hello AirNav??

[Tramline]

Hi,

Blue skies, plenty of A/c but dodgy alert function.

My alert function seems to work only 80% of the time, I saw VT-ALH, G-FBEN and ZZ175 about 30-40 mins ago - both on my alert watch VT-ALH and G-FBEN were entered as registrations and ZZ175 should have alerted using the 43C* Mode S alert, however neither 'alerted' however, the other 80% today have alerted both on reg and or Mode S. 

I've checked that boxes are ticked and that the info is entered correctly - can anyone offer reasons why alerts are so 'iffy'?  It's disappointing because the alert function is vital for me.

Any ideas?  Airnav- Hello - can you comment?

[Tramline]

VT-ALQ just failed to Alert
VT-JWD - DID alert!

Same format, both in system only one Alert.

Why?