It sounds like you are doing everything correctly. If you had not tried to mate her to another male already, that would be my first suggestion.
The other big possibility is that she is not ovulating and will not become pregnant. It does happen, although it is very unusual.
When her next heat occurs I would try another male. Allowing the male to mount at the time you indicated is the correct thing. If she fails to get pregnant again, you can keep trying the same thing (because sometimes these things do happen), get her spayed so that you no longer have to monitor her when she comes in heat, or take her to your closest veterinary college for a full work up.
The vet team at the university might decide to examine her uterus and ovaries with an endoscope. They might try hormonal treatment to bring her into estrus and then flush her uterus to examine any eggs that are shed.
This is going to cost you money. If the potential gains from the puppies is not worth it then it would be better to have her spayed. If you are a hobby breeder then you may just be looking for one of her puppies, in which case the costs are not the main issue. You really have to decide.