HaShem's blessings are constantly coming to us but they are not bound by time, so there is no way to be sure when and how they will reach us. More importantly the blessings may reach us at inappropriate times so that we go hungry sometimes and have overabundance at others.Remember that HaShem is always giving to us. It's we who are not always open to receiving.
Bitachon, trusting in HaShem, is looking to HaShem for what we need now. By the very act of looking to HaShem we create a time-bound vessel in which to receive HaShem's blessing when we actually need it.
He brings support for this idea from a passuk we say before making the blessing on bread: עיני כל אליך ישברו ואתה נותן להם את אכלם בעיתו - the eyes of all look to you, and you provide for them their food in its time. It is only because they first turn their eyes to you that they are able to receive their food in its time.