Problem monitoring beacon Frames from VAP

Hi!

I have this setup:
One VAP (ath1) in ap mode and one (ath2) in monitor mode.
Both seem to work. If I use a sniffer like airodump-ng on ath1 I can see 
a lot of traffic and - what I'm interested in - beacon frames from all 
available APs. But I can not monitor a single beacon Frame coming from 
my own VAP ath1.

Can you explain, why? Can I do something to monitor/record them?

I tested this with Netgear WG511T Cardbus Card and 3Com 3CRWE154A72. 
Both show the same result.

Thanks in advance!

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