The way to tell is kind of complicated as you will need to start fop2 in debug mode and look at the manager events just in the moment you see a queue button show empty members (not sooner, nor later, as the log will be huge and hard to follow).

To start fop2 in debug mode interactively (so you can stop it when the error happens), you can do this:

service fop2 stop
cd /usr/local/fop2
script capture.log
./fop2_server -X 511

You will see lots of things flowing on the screen. Log into fop2 and monitor your queue buttons, once you notice a queue button show no members, do this:

ctrl-C
exit
service fop2 start

You will end up with a capture.log file that will let us inspect/analyze what could go wrong.

Best regards,

fir has 0 calls (max unlimited) in 'rrmemory' strategy (3s holdtime, 96s talktime), W:0, C:3208, A:447, SL:0.0% within 0s
Members:
Local/841@from-queues/n (ringinuse disabled) (dynamic) (Not in use) has taken 40 calls (last was 144 secs ago)
Local/875@from-queues/n (ringinuse disabled) (dynamic) (Not in use) has taken 17 calls (last was 144 secs ago)
Local/466@from-queues/n (ringinuse disabled) (dynamic) (Not in use) has taken 44 calls (last was 308 secs ago)
Local/124@from-queues/n (ringinuse disabled) (dynamic) (Not in use) has taken 2 calls (last was 175 secs ago)
Local/279@from-queues/n (ringinuse disabled) (dynamic) (paused) (In use) has taken 16 calls (last was 221 secs ago)
Local/275@from-queues/n (ringinuse disabled) (dynamic) (Not in use) has taken 2 calls (last was 195 secs ago)
Local/207@from-queues/n (ringinuse disabled) (dynamic) (Not in use) has taken 25 calls (last was 225 secs ago)
Local/816@from-queues/n (ringinuse disabled) (dynamic) (Not in use) has taken 43 calls (last was 199 secs ago)
No Callers

sec has 0 calls (max unlimited) in 'rrmemory' strategy (2s holdtime, 184s talktime), W:0, C:1291, A:239, SL:0.0% within 0s
Members:
Local/390@from-queues/n (ringinuse disabled) (dynamic) (Not in use) has taken no calls yet
Local/387@from-queues/n (ringinuse disabled) (dynamic) (Not in use) has taken 24 calls (last was 29 secs ago)
Local/690@from-queues/n (ringinuse disabled) (dynamic) (paused) (Not in use) has taken 11 calls (last was 20 secs ago)
Local/611@from-queues/n (ringinuse disabled) (dynamic) (paused) (Not in use) has taken 1 calls (last was 4 secs ago)
Local/794@from-queues/n (ringinuse disabled) (dynamic) (Not in use) has taken 1 calls (last was 78 secs ago)
No Callers

thi has 0 calls (max unlimited) in 'rrmemory' strategy (12s holdtime, 93s talktime), W:0, C:980, A:1163, SL:0.0% within 0s
Members:
Local/319@from-queues/n (ringinuse disabled) (dynamic) (paused) (Not in use) has taken 41 calls (last was 14 secs ago)
Local/983@from-queues/n (ringinuse disabled) (dynamic) (Not in use) has taken 38 calls (last was 55 secs ago)
No Callers

At this moment in fop2 queues all and fir looks empty, but queues sec and thi still looks fine.
Changing label did not help.

asterisk -rx "queue show"

On those moments you see the queue members empty in fop2 ? Anyways, try to always set a label for fop2 buttons instead of leaving it empty, an empty label in a button will actually disable it.