I need help

I work in a store-kiosk... as in, a Starbucks situated inside a grocery store. Typically, only one person is scheduled to work alone, which makes closing difficult, especially for me because I'm still fairly new (I just started working 2 months ago). My closes have been extending past 7:15PM, the time im expected to punch out. And although my co-workers keep jokingly saying that I'm intentionally working overtime (which isn't the case; im really trying to finish on time), it feel like managers also feel the same way. Are there any tips you guys can offer, because I've tried cleaning up 20 minutes early and restocking during my 30 minute breaks, but nothing seems to be coming of that. Any advice is appreciated. Thanks.