I had 3 children in my first gen family and now my second gen family has 3 children but I think I've tried 6 times for the parents to have more kids but it doesn't work.

No, I don't think it makes any difference. I had four in my first generation, five in my second, four in my third then six in my fourth. You just need to keep trying. If they say it's to soon to try again, leave them alone for awhile but if they argue, you can keep trying until they try again. Hope this helps.

Have you tried the baby boost? How old are the parents? after 35 years most of them will require a babyboost to have a baby. also make sure they have showered, been fed, and are healthy. also from my 'notes' on my games i have noticed that they are more likely to have a baby if they are not tired/exhausted. Hope this helps!
(Edit) You can make them try for a baby every 15 minutes. Unless they say No its too soon!

That's happened to me twice. One was 52 and one was 45. I just kept trying till they had a baby. I must have tried ten times on each of them. I know they won't have a baby when they are hungry. They will say, "I'm too hungry for this." lol

In one of my families i had 6 kids in the 1st generation, 3 in the 2nd, 1 in the 3rd, 3 in the 4th, 4 in the 5th and 1 so far in the 6th.

The difference I get is a ton of kiddos one gen then Less kids for a generation. Then the one after has a few kiddos and a couple orphans. the one after that has a ton of bio kids again.

no because i got 6 kids on the first gen in all but 1 of my families. it took till the 5th gen in that family to get 6. like everyone says, just keep trying!

The number of children depends on how quickly your peeps want to try for another baby (i.e. two "Definitely"'s will try much more quickly than two "No Way"'s) I try to make sure all my parents are either Definitely/Maybe/Would Be Nice (in the Wants Kids section) if I want them to have a lot of children.