MacBook Pro battery not fully charged sometimes. Need tips
This is by design. It's not helpful to fully charge a battery, and then use a little bit (say 2 or 3%) and then top it back up again. So purposefully, if you have fully charged your battery, it will not start to recharge it until such time as it has lost sufficient charge for a recharge to be worthwhile. It's all about charging your battery efficiently, resulting in minimal wear and tear where possible.

So, to try to illustrate it by example:


You start with a flat battery, and charge it. It gets to 100%
You move desks or something, resulting in a few minutes of battery usage, dropping it to 98%, then put it back on power
The system decides it's not worth topping up 2% worth, and just sits there.  The green light probably shows on the Magsafe to indicate that it's not even trying to charge, likewise the menubar icon shows a plug indicating full charge, rather than the lightning bolt charging indicator
You use some more battery, say dropping it to 92%, before putting it back on charge.  The system now decides that 8% is worth bothering about, and starts to charge.