WiFi card can see and connect to other networks, but hostapd doesn't broadcast anything

[Sam Jaeschke]

I am using OpenEmbedded on an embedded system. I had a copy of the
build recipes from towards the start of last year, and got my ath9k
WiFi card and hostapd v2.2 working. It came time to update all the
scripts to the Poky Jethro 2.0 release (from Nov 2015 -
http://cgit.openembedded.org/meta-openembedded/tree/meta-oe/recipes-connectivity/hostapd/hostapd?h=jethro).
Some PCIe and WiFi things needed updating to make that work, so when I run:
> iw dev wlan0 scan
it showed me the list of networks it could see. So far, so good.
However, hostapd was upgraded to 2.4, and it stopped working. All the
messages it output was fine and normal, and there was no indication
that anything was wrong (I diffed the initialisation output and
hostapd_cli 'status' with the earlier working version) - however other
devices couldn't see the SSID appear in their network list, so it's as
though nothing is being broadcast. The WiFi card still works in
standard (non-AP) mode.
I kept all the recipes at the Jethro Nov 2015 release, except I
grabbed all the recipes and configuration I had used for hostapd 2.2
from before.. so exactly the same code and config. I tried running it,
and it still behaves the same way - everything it outputs is fine, but
there is nothing broadcast that can be seen by other WiFi devices.

1. Is it possible that WiFi kernel drivers, etc. can work fine in
normal mode but not AP mode?
2. Any ideas on how I could track down this problem?
3. Any ideas why hostapd pretends everything is fine?!

Many thanks,
Sam