amavis av_scan randomly skips av_scanners

[Martin Thomas Swaton]

Hi together,

I am facing a strange behaviour and am searching for a solution.
I have configured four av_scanners:

1 a small pl script that searches for some headers in the mailfiles
2 another pl script that searches for vba code.
3 sophos savscan
4 clamd


All work well and give clean/infected state back.

But sometimes (maybe when there are more messages at the same time) 
amavis just skips some of the scanners without any note.

In the logfile at level 5 I only see which scanners are used (run) but 
no note about the others!

What I do see, that the first two seem to be skipped and the other two - 
maybe not.
Sophos and clamd also are in the backup list so this may be, why they 
are not skipped? But would that give a note, if the backup list is used?

Any ideas what is happening here?

thanks,
martin