The nature of an optical flash slave is that it needs to see the light. If they made the slave too sensitive, it would fire when you didn’t want it to. So there are several methods to get it to see the light. You could put a device near your firing strobe, like an index card, mirror or foil to redirect a small portion of the firing strobe in the direction of your remote strobe. You could also position those same reflectors near the sensor for your receiving strobe to collect some of the light & bounce it back on the optical slave. You could get a coiled sync cord & use an optical slave at the end of it, positioned facing your firing strobe & connecting the other end of the sync cord to your remote light. You could also attach a flash to your hot shoe & either use a dome cap on the top to disperse its light or attach an index card to it, in order to redirect the flash toward your remote unit. Most pros will make their life simpler by using a radio remote system. There are several available in the market, such as pocket wizard.