Starting with iOS 7, Apple has restricted the possibility of using third-party accessories to charge the iPhone or iPad, displaying the notification in the operating system This Cable is Not Certified and May Not Work Reliably. Normally it is displayed only when you use a counterfeit charger or cable to connect an iDevice to a computer or charger, but unfortunately it also appears on original accessories. Now if you wonder why it could appear with an original accessory, there are some simple reasons that, once removed, allow fast charging.
The first reason is based on the use of an accessory that has not been certified for use by the Apple company, and in this situation, the only way to solve the problem is to buy an original accessory. The Apple company offers the possibility to buy an original charger for iPhone and iPad for only €10 if you leave the counterfeit one in the store, but you will also need a new Lightning cable to synchronize or charge without problems.
The second reason could be related to the state of your accessory, more precisely it could be hit or even partially destroyed, so it is not correctly detected by your iDevice. In this idea, the recommendation made above remains valid without discussion, but if you do not have an Apple store or representative nearby, it is advisable to buy a third-party accessory, preferably from a company certified by the Apple company.
Finally, there remains the possibility that the charging port of your terminal contains dirt that prevents full contact between the cable and the charging port. In this situation, an efficient cleaning of the charging port is the only solution to solve your problem, replacing a accessory not necessarily being necessary.
From my point of view, using counterfeit accessories is extremely dangerous and can even bring you death while charging at an outlet.
