The tablet might be drawing too much power for the external battery to keep up, especially if the tablet is on and if the battery wasn't designed mainly for tablets (though most of the higher capacity ones are rated at the higher amperage 2A for quick charge/tablet charging).
Which battery and tablet are you using? Try turning the tablet off and see if it charges (wait a while and then check the battery percentage). If it is charging slowly, then it's probably just not ideal for the tablet.
EDIT: See you mentioned the ipad. If it's a full 10" iPad and not a mini, it probably is draining power faster than the external battery can provide