We are trying to breed our female lab but are having problems with the males. Can you help me with a few questions?

I have a yellow lab which I'm trying to breed for three days. My family decided to breed her and vet told us that the fertile days are 9-13th. We tried the tenth day to breed her with another 2 year old lab, but he didn't know what to do.He licked her and did all other stuff, but he didn't hop on her. We were trying with that lab for three days in a row, but nothing happened so we tried with another 5 year old black lab. His owners said he mated before, so we thought it's going to be easier.

We went two times to his teritory(in a park in front of his house) but he was intersted even less in our female then the first lab. We were really desperate,so we nearly gave up on breeding her this year. She is 5 years. 4 days ago we noticed other dogs in front of our house. And they were interested in our female. It was her 17th day. We decided to give it another shot (even tho we expected nothing to hapen). But this time black lab hopped on her multiple times. She still moved her tail so we thought maybe her days are now. He hopped on her, but didn't finish the job. He would actually stay short time on female and than hop down. We were trying with him for three days now but there's no progress. He hops on her, but they don't get a tie. These were her 18th and 19th days.

I read your answer on 34. question on breeding and because I can't do that and if it's necessary I thought I should call a vet. The vet said that (because it's her 20th day he won't come and that we should try for 7 months with artifical). What's not clear to me now is if these are not her fertille days why didn't the male hop on her the 12th day? Why is he hopping now? And why did the female let him hop on now? The female has it on 7(not 6 months) and it approximately lasts for 4(not three weeks), so I thought is it possible that her days are something like 15-20th? Is the vet right? What should I do now?

Thank you.