trying to conceive HELP anyone

Well I was trying to conceive for
Some strange reason it didn't work I always get my period at the same
Time and I have follow the pattern of my menestral cycle so I can know when I'm fertile but it didn't work and we did it 4 days straight and on the main day that says you're most fertile didn't work.... had a strange period then after was spotting them left came back
Two days after left again and then boom a week later got my period I have kids and he only has one but I want to have my last before I tie up what can I do ??