Brit Kit

Does anyone know why I sometimes get offers to watch ads for extra stuff, and sometimes I don't? For example, occasionally in the arena after a fight there's a "watch video" button and sometimes when I watch the video I get more gold and extra arena coins and level points, and then sometimes I click it and I see a message like "no awards available at this time". Most of the time the button isn't there at all. If I watch it and awards are available, then there are other benefits, too, like watching ads for a free item when I buy item boxes, or 20% more hp in campaign fights, or getting 2 tokens after losing a campaign fight. I switch back and forth between playing on my Windows desktop and my old, crappy tablet. I have no idea what causes it or prevents it from happening, any theories? Thanks, BK