doesn't always charge to 100%

I'm new to all this so bare with me. At first I could charge it to 100% and now it only charges to less than 50%. I read on line that this could be fixed by resetting it and I did and it would go back to charging 100&. After a few successful charges it returned to only charging to 50%. If I don't follow the reset instructions it won't charge completely. I've even left it on chargefor an extended amount of time but it still doesn't fully charge. What can I do to fix this? I appreciate your help in this matter.