Ok. Sorry for the delay, I am home with my kids today which means sometimes I get pulled away from the computer!!!
Fleas can be a very frustrating issue, and it can take a multi-faceted approach if you have a severe case of them. It sounds like you have already done many things that we recommend, so that is great (the cleaning of your house/vacuuming/etc.), I would also vacuum your hardwood floors, as sometimes the flea eggs/larvae will actually fall into the cracks between the floorboard (they are microscopic in size so you wouldn't see them), and then hatch later on, so vacuuming the wood floors and then disposing of the vacuum bag as you did would be a good idea as well.
Frontline can be a great product, but some owners have noticed fleas becoming resistant to the frontline (and some other topical products), so sometimes we will have the occasional flea that it doesn't kill. Usually most of them are killed by it, but unfortunately it isn't an instant kill with this product. It can take 24 to 48 hours AFTER coming into contact with the frontline for the flea to die (the slow moving one you found was probably one of these that had come into contact but wasn't quite dead yet). So if there are fleas that hatched recently in your home, and they then jump on your kitty and you see them in those first few hours, they are going to look like healthy fleas. There are some products that are oral flea preventatives that work a bit faster. My favorite is a pill called Comfortis. They work a bit faster (fleas usually dead within 4 hours versus 24 to 48 hours). This issue is this product is only available from a vet or with a prescription from a vet. If your cat has been to a vet within the past year, they should be willing to prescribe it to you without your having to bring your kitty in, so that would be the fastest option for you!!!
The other things you could consider would be treating your apartment with either a spray (you spray the floors/furniture) or a fogger. You can pick these up at your vet's office or at a hardware store. The issue it, you will have to remove your kitty from the apartement for part of the day while you are treating the place, so that may be tough depending on your situation. If you cannot do that, but have separate rooms in your apartment, you could put the kitty in one room while you spray the other room and wait several hours for it to dry completely (you couldn't really use the fogger with the cat in the apartment as the doors probably wouldn't keep it out).
You will probably need to keep your kitty on a monthly flea preventative now since she has had fleas once. If someone in the apartment complex nearby has a dog, it is quite possible that dog has brought fleas into the building and they will continue to be a problem if you don't stay on top of it- makes it tougher than having a house you know no one else is entering/that you can control completely.
Also, it is not uncommon to have a resurgence of fleas about 3 weeks after your initial treatment because of the flea life cycle. Any eggs that were in the environment that didn't get vacuumed up may hatch (there is nothing that will kill these in this form) and then those fleas will have to come into contact with the frontline (or you can retreat the environment as well) before they die. Then hopefully they will not have a chance to lay any eggs and we should have the issue more under control (here is an article which gives some info on their life cycle: http://www.veterinarypartner.com/Content.plx?P=A&S=0&C=0&A=3237).
So to make a long story short, the fleas you are seeing aren't unusual even after treating with frontline- there is a small chance some are resistant, and a greater chance they just haven't died yet. If you want a faster product, I would call your vet for comfortis. I would vacuum the hardwood floors as well, and consider treating your home. You may need to retreat your home in 3 weeks. And then keep your kitty on the monthly preventative.
Please let me know if you have any questions about this information!!!