The yotes are active mainly at night due to the heat. Your best bet would be very early and very late in the day. I like rabbit and rodent distress on my Foxpro, you may have to get aggressive to get them interested. The evening is always better for me, right now they are working night shift. Remember not to shoot the foxes if they come in, they are out of season. You need to be as scent free as possible and have full camo including hands and face. The odds are that you have called in coyotes, but they did'nt come into view or you got busted before you saw them. Your setup is very important because your movement will be limited when your calling and it seems like they always come in behind you. Coyotes tend to circle downwind of the call so be prepared to shoot in that direction, but this will be determined by the layout of the terrain also. Try to think like a coyote and spend that extra time finding a good set location and it will payoff in the end.
Good Luck