The French intelligence and counter terrorism units are either not doing their best or these terrorist are getting much better at covering their tracks.
If it is a small group of just half a dozen people you should be pretty much invisible if you act a bit clever. Coordination can be done via throw away sim cards and personal meetings so you don't really need to communicate that much.
You forget the fact that the communication channel is always being monitored, but in this sad case, nothing was caught. Someone has already smuggled the weapons in before hand, just like how the bomb was able to made it to the plane early this month. Someone must have been tipped and bought the weapons from possibly local gangs and bam...