Hi,
Assuming the unit is set to at+cops mode 0, you should avoid setting it to 0 again as it can cause unforeseen side effects. If it is set to 0 then the correct procedure should be to leave it to re acquire the network, the algorithms are designed to do this as fast as possible. How long are you leaving after the unit has been taken out of the bag to re acquire the network? It should be pretty quick to see it.
Does performing a reset help? Because the unit resets so quickly it might be faster than the below although I would question why it does not re attach first of all.
If you want to take an active role and force a scan for which you can see the results then you could try sending at+cops=? which forces a scan across all technologies and bands that are enabled, this can take a couple of minutes to complete depending on how many cells the unit can see (if it can see a lot then it has to listen to all of them).
Regards
Matt