Terry is correct. The issue is the distance from the Barlow to the chip in your case, and more so with the 5x Powermate which for some reason you didn't report back on?
Try this link
http://www.televue.com/engine/page.asp?ID=42#MagChart
It shows the 5x Powermate increasing it's power with the greater distance Powermate to Chip. The 2.5x Powermate actually decreases slightly.
Gary