genomic DNA digestion - EcoRI+MseI - (Nov/23/2008 )

I'm trying for a long time to double digest genomic DNA of a Tamarix tree with EcoRI and MseI. I know the enzymes are OK, but I still can't get a good digestion. I think the problem might be polyphenols or carbohydrates, but all the DNA extraction techniqus I tried (and I tried a lot...) don't seem to affect the digestion.
Does anyone have an idea how to improve digestion or a good way to clean the DNA?

try digesting for longer (overnight or even longer) or digesting at 4 degrees for a long period.

Thank you, but right now I digest for 10 hours, 37 degrees. I tried different time periods and saw no improvement. The problem is probably something that inhibits the reaction, but I'm not sure what and how to get rid of it...