Probably a stupid question with a simple answer...

I got a Flashpoint II 320 M and it has done great. If I connect the flash to an adapter pc socket then the images are great, but if I disconnect and use the on camera flash to "pop" the flashpoint then it comes out very underexposed. This is confusing because I use an off board speed flash as well that I trigger with the wireless flash setting and it is great? Any help is appreciated the problem is that I don't want to be "hard wired" all the time.
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"We can do no great things, only small things with great love" Mother Theresa