E-cigarette batteries are consumable and have an approximate lifespan of around 2-3 months subject to user care. If your battery is more than a couple of months old it may need replacing.
However, before looking into buying a replacement, you can try the following:
Check that the threads of the battery and USB charger are clean. Fluff or a build-up of dirt on the threads will prevent a proper connection and may also damage battery cells. You should regularly clean the battery thread by wiping it with a tissue.
If an e-cigarette battery is stored in a fully discharged state it will often fail to recharge. Storing or placing an e-cig battery next to magnetised objects such as keys or sat navs, e-readers and mobile phones may damage the battery cells. If the battery fails to charge, you will need to order a spare battery.