When Pota was operational during NDA's time, some of the most audacious terrorist attacks took place - Parliament, Akshardhaam, Reghuram Mandir etc So attacks will take place, with or without such laws.
The problem is more deeper. An advisory warning will not help state governments. In Karnataka the Police came across evidence of a blast in Channapatna - 50 kms from Bangalore, a day earlier and recovered a car full of explosives. The State government did nothing. This despite a central government advisory to be on high alert - 3 days before the blast.
The Centre gave an advisory warning to Gujarat 3 days before the blast. Despite Bangalore blast, the State government did nothing. The Gujarat government instead commenting on the Bangalore blast boasted on TV that such terror strikes would not hit Gujarat.
Instead of mainstreaming disgruntled sections of society - Naxalites, Muslims etc, the BJP and Hindutva follwers isolate them further. As long as this happens, POTA or even a stronger law will not help.
The Gujarat government complain that Central warning was not actionable. OK. Now that the blast has taken place, can they crack the case and bring the terrorist to book. At least if they do that, there is some hope.