Just because they can find them doesn't mean they'll breed. Females can and DO fight off males. I've got a tank that's had stock in it for 7 months that not one shrimp has bred in. It happens. I've watched saddled females fend off suitors the entire few hours they are breedable and harden right back up with the saddle in tact (after so many times the saddle just reabsorbs).
As SS mentioned not all females have a stripe down their back. It's just part of coloration/pattern, not a true way to determine sex