Does Preen work at preventing weeds

I have always had weed problems but not ever as much as this year. Maybe it's because I'm a year older and the heat and humidity is terrible. In most of my beds, and soon to be all of them, I have good quality mulch fabric down and about 4 or more inches of mulch. I find it hard to even keep the crab grass and other weeds from taking hold in the mulch, just in certain spots and not the entire bed. Is this from grass clipping from when I moe?

I need to find a better way of controlling weeds and spraying in these areas is not an answer for close plant areas. Does the weed control products work?

Kathy