This has happened a number of times, according to stitchers who buy the kits. My understanding is that you should contact Dimensions and explain to them of your floss shortage, what kit name is and request the colors you are short . I've heard from other stitchers this seems to be the action to take and that Dimensions, who creates the kits, responds by sending you the floss you need. How long it takes for you to receive the floss is a good question when you call or e-mail them. If you have the written directions that came with the kit that should have a contact phone number or e-mail address in which to contact them.
I am not an employee of Everything Cross Stitch, but one of the stitchers that saw your chat. :)

Hello,

Bermuda is correct in her post. Dimensions uses their own brand of floss so if you are shorted floss in their kits or if you need more, you will have to contact them and request it. Here is their contact information if you need it:

https://www.simplicity.com/contactus
Monday to Friday
8:00 AM to 4:30 PM CT
1-(800)-794-5866

Hope this helps!
~Kristi @ ECS