DNA don't migrate...

[Jukka U Palo]

One stupid question again. Recently I started to extract DNA from shrimps
(which, btw, seems to be difficult - any suggestions?). When I'm checking
the DNA in agarose gels, a great portion of the DNA stays in the wells.
What is the reason for this? First I thought it is due to proteins and
other impurities, but the samples behave in a similar way after second
phenol/chlorofom extraction, too. This happens with some of the samples, and
even seems to be somewhat species-specific phenomenon. These samples
still work reasonably well as PCR templates, but I'm just curious. Any
similar experiences?

Thanks, Jukka