Getting Stores to pay

Hi All
This is my first blog ever, so I have alot to learn, especially the lingo.
I have been selling my clothing wholesale for 6 years to small boutiques. In the last two years, about 75% of the stores have delayed payment past the 30 day net period. I am finding myself spending more time on collecting funds than designing..and it has impacted my cash flow so much that I am only doing retail shows this year and selling on line to pay off the debt.

I do take credit cards, but even that is hard to collect...they only give me dribs and drabs.
Any advice?

My web site is www.alextex.com
I sell on Esty: http://www.etsy.com/shop.php?user_id=5243667

and www.smashingdarling.com/store/1657

Would love to hear your opinions and ideas!
Janetexcat