Trouble backfilling certain IQFeed tickers in scan/exploration

I have a scan that requires the "Wait for backfill" box to be checked. It works except for three tickers that never backfill on their own:
@C#C
@S#C
@W#C

When I run the scan, it just hangs on each of these tickers. To fix this, I select the ticker in a chart, and click "Force Backfill", and in a few seconds the scan resumes.

I'm not sure where to turn for support on this. Is this a plugin issue, AmiBroker issue or an IQFeed issue?

Neither AmiBroker nor the plugin differentiates between symbols. They treat every symbol exactly the same. Also there is no difference between accessing via scan or chart. If it was plugin issue it would not work for chart. Plugin does not hang. It WAITS (but asynchronously, non-blocking, so entire UI is fully responsive) for IQFeed to respond with the data. If data never arrive it may wait forever.

Ah, thank you for the clarification. I should have said the plugin waits, not hangs.

And this is a key insight:

So it must be an IQFeed issue and I'll take it up with them.