Well I would assume you had location services on so that is a given.
Besides that make sure the address for that Target matches what the Maps app thinks is Target (have the 4S locate you at that location and compare that address to what you have in Contacts for Target).
I have had to add a second "GPS Address" to several contacts, including my own home, in order to better get location reminders to work.
In the case of my house, Maps shows my location perfectly, right on the corner. And I can even see when in sat view where I am in my house. But if locate myself in Maps it says the address I am looking at--my own house--is a few houses down. So if I set a location reminder and did pass the house a few doors down--because I went the other direction--the reminder would not go off.