TATP is fairly easy to make with obtainable chemical materials, but once again it can easily detonate during the mixing process. Another problem is the mixture is time sensitive, it normally starts to lose effectiveness over even a few days (I have fired off C-3 explosive made before WWII without a problem). Do not expect TATP to work even after a week in most cases. So if you look at the timeline of the Brussels attacks, the plan was probably developed over the last few weeks or months. When they decided it was time after the capture of the last Paris bomber, the delay was probably having the TATP made up.
Rich Roth via CTI Consulting http://ift.tt/1q0otqq