New 26" inner tube different size to old 26" inner tube
Check the label on the tube - does it say '26 x 1 3/8' by any chance? If so, this is actually a completely different size than the standard MTB 26". Get a tube that says something like 26x2.1 or 26x1.9 - the point is, the tire width should be expressed as a decimal, not a fraction. See Sheldon Brown's tire size chart.

Yeah, I know, it's stupid - I didn't come up with this system.