Tube amp messing up

So my tube amp sputtered out at practice today any help would be awesome. I thought it was a tube (easy fix) but after going threw each one I found that's not the issue. When I pulled the amp out of the box I put it upside down to see if anything came loose plugged it all back in upside down and it worked great. Figure well maybe it was a tube after all. Put the amp all back together pluged it in again and nothing...... Well then I flipped it over just for giggles and it starts working again as you start flipping it back upright it goes out. If anyone knows what could be causing this issue that would be great. Thanks