Which channel are you monitoring, and in what language?  I use

       AT+CNMI=1,0,2,0,1;+CSCB=0,"50,200","0-15"

This listens specifically (in all languages) to channels 50 and
200, as these seem to be the commonly used channels for
"location information".

The "AT+CNMI" command gives unsolicited "+CBM:" results.  That
is to say, it will dump a "+CBM:" result when and only when the
cell broadcast changes (ie, when the modem camps on a different
cell).  But I would also expect a single "+CBM" result within
about 10 seconds of giving the above command.

I'm not sure what you mean by "for a very short duration".  If
the cell doesn't change, you should get no further "+CBM:"
results from using the command once only.

John

Are you perhaps trying to control the times at which you get to
see the location cell broadcast (CB) message on the serial
port?

Bear in mind that CB is a broadcast SMS, and is sent
periodically (but very often) rather than continuously.

Repeated use of the "AT+CNMI=1,0,2,0,1" command changes nothing,
so failure to get a new message indication (a "+CBM:" result)
is no surprise if the modem hasn't changed cell.

If you want to read the latest CB message on demand only, I
suggest using

        AT+CNMI=1,0,2,0,1;+CSCB=0,"50,200","0-15"

(modified to your requirements if required).  Then wait for the
"+CBM:" result.  Then turn direction to the serial port off
again with something like

       AT+CNMI=0,0,0,0,1

before turning it on again when required.

Personally, I leave it turned on, and use the incoming "+CBM:"
results to tell me that the serving cell may have changed
(confirmed by "+CREG" results).

John