Why don't my shoes stay tied?

Seriously, I must tie my shoes five times a day when I am at school or work. They just don’t stay tied! I have been tying my shoes for a long time now, and I tie a mean bow if I do say so myself, but for some reason they always seem to come untied. I know for sure it is not an issue of the laces being too long and my stepping on them, it is also not an issue of them not being tied tight enough. But for some reason my shoes always come untied! I really don’t want to resort to the double knot here, that seems more suited for my 9 year-old!