How old is Mochas? If you pull the hair too tight, you can lose blood circulation to that area, and hair loss will occur, but it doesn't sound like that's the problem. Is Mochas still a puppy, maybe the hair is just still thin. Here's a great site that has some tips on getting you dog to wear a topknot, and I also recommend the latex bands, they are easy to remove, but stay in place very well.
The Diva-Dog Bowtique