Help! TTC but not working.

Me and my husband been TTC for 2 months but it’s not working. With my 2 kids before we have only done it once then I was pregnant. But now it’s just not working and I was wondering if I was just calculating my period wrong. So do you guys mark off where you stop bleeding heavily or when all of it has ended like it’s coming out brown. Cause I’ve been marking it when the red has ended. Sorry if that doesn’t make sense.