The Facebook ads learning phase is the period of time Facebook's algorithms try to work out how to generate the best results possible for your campaigns. They serve your ad to different people within your target audience in order to discover who is most likely to take your desired action.
The learning phase takes about a week to optimize. But This time frame will restart if the campaign is modified. This means that each time you modify your campaign you are loosing the optimization progress you had made, and the Facebook algorithms will start over trying to figure out who to show your ads to in order to get you the best results.
For that reason we strongly suggest Waiting to edit your ad set until its out of the learning phase. During the learning phase, performance is less stable, so your results aren't always indicative of future performance. By editing an ad, ad set or campaign during the learning phase, you reset learning and delay the delivery system’s ability to optimize.