iPad charging issues

If the adapter that you are using is a low power adapter, the use of the iPad while charging could be using more power than the adapter is able to produce, thus you are using that power plus battery, which would explain why you are not charging but actually losing charge. The previous poster was correct that it is better that you use the higher power iPad power adapter, but if you cannot, then you will need to charge the device while not using it. It isn't something wrong with the iPad, it is the power adapter.

If you are referring to the smaller 5w adapter in the photo below the one you pictured, while that will charge the iPad, it does not do so good under a load. The iPad should be charged with the 12w adapter.

What kind of adapter are you using? IE the one that came with it, iPad, or 3rd party?

So why does it not charge when I do use it?

I just explained why that is happening, and if the battery is lower, the more it matters. Since you are using a 3rd party adapter, it very well might not be delivering a constant power output either. Way to many variables that cannot always be explained when you are not using genuine Apple products. The same thing will happen to me if I plug mine into my computer and try and use it.

Does it say the wattage on the adapter?

It is not Apple.

I strongly suggest purchasing an Apple 12 watt adapter; 3rd pay adapters can potentially harm your device. Worth it to protect your investment. They are available at Best Buy for 15.99.