Evagrios of Pontus: Sometimes it happens that when you start to pray...

Sometimes it happens that when you start to pray, you find you can pray well. At other times, even when you have expended great effort, you may find your efforts frustrated. 

This experience is to make you learn that you must exert yourself constantly, for having once gained the gift of prayer, you must be careful to keep it safe (29. Philokalia 1:179).

Evagrios of Pontus