Lightning network routing failure rate
Indeed, the current state of the channels is not announced (it is allegedly hidden for privacy purposes). Therefore, A does not know if a route has enough capacity until he tries to make the payment throught that route.

A recent study claims that success rate is around 80% for USD 1.2, but only about 1% for transactions larger than 200 USD.

Related question: In lightning network, is the balance publicly anounced in realtime?