This latest failed attack is puzzling. I can think of scenarios that might explain it- the bomb-maker was incompetent- he deliberately sabotaged the mission- but they're all a bit wild. Four dud bombs in a batch of four is verging on the surreal.
The good thing is the police now have four half-exploded bombs to submit to forensic examination. They also- pretty soon- are likely to have four live bombers to interrogate.