Just so I know you have all the information on the heat cycle, I am including it here. A few key things is that the female needs to be in the right stage of heat and also needs to be brought into the males territory for mating and not the other way around. It sounds as if you have done this.
To breed successfully you need to understand the heat cycle. I have explained it below and included a site that has a pictorial guide so there is no mistake what stage of heat your girl is in.
Female Dogs go into heat at any time between 6-12 months usually. The average heat cycle for a dog is approximately 3 weeks but for some dogs it is longer and for others it is shorter. Some as short as 10 days and some as long as 4 weeks or more. Usually your dog will go into heat every 6-7 months though again each dog is different and can go back into heat as early as 4 months or not for 12 months. Most however are average.
During the first part of a dogs heat period you will notice swelling of the vulva, possibly an increase in urination and bleeding. If there are male dogs around you will notice them hanging around her. This stage generally last about 7 days and she will not allow males to mate with her during this period The second part of the heat cycle is when she will accept a male and breed. Usually bleeding has stopped or become straw colored at this point. This can last from 4 to 21 days though the average is 7 days. Many expert say the 9-10th day of heat is the optimum day for breeding. It is during this period of time that you will want to allow mating. Most breeders allow the dogs to mate every other day during this period.
Then during the last stage of heat she will be less willing to breed as she is going out of heat. This stage can again last from 4 to 21 days as well but averages 7 days.
Each dog is different so some evaluation by the owner is necessary. The whole heat period should be approximately 21 days but as stated can be longer or shorter depending on the dog.
Here is an excellent website with pictures to illustrate a female in different stages of heat.
It sounds as if there is still a few more days, but you are getting close to the time when she will not be receptive anymore. Sometimes the problem is a female moving her hindquarters at a crucial moment or walking forward while the male is mounted. You can try leashing your female so she doesn't do this. I have done this with first time mating females with good success.
If you still don't get a "tie", then you might need to enlist another person to help. One to hold the females leash and a second to help guide the male's organ to the opening. Generally, it will appear the male is making a half-hearted attempt until his organ hits the vulva. Usually when that occurs, you will notice an almost frantic pumping and this usually signals that the mating will be successful and a tie will occur.
Left to their own, it is not unusual for a male to attempt to mate by mounting and dismounting for 45 minutes or so before actually succeeding in a "tie".
Pitbull breeders sometimes use breeding stands which you can see here.
If all else fails, it may be necessary to have artificial insemination performed. Here is an excellent site on this.
And purchase kits from here:
I hope you find this information helpful.