Modern mobile phones use various combinations of GPS, Wi-Fi, and cell tower triangulation to estimate a phone’s location. Depending on which of these signals is available, the reported location can be very accurate (down to 10m/30ft if GPS is turned on and you are outside in the open where satellites are visible) or wildly approximate (somewhere in a 5km/3mi circle).
Foursquare uses these location estimates to deliver lists of places, tips, and friends that are near you. If a place or a tip you expect to see isn't in the nearby list, it may be because your phone thinks it is somewhere other than your current physical location. The black status bar near the bottom of the Places and Tips views shows the approximate address where your phone thinks it is. If this address seems wrong, you can force the phone to refresh its location by pulling down on the current view and releasing.
If the location is still wrong after you pull to refresh, you could try to make the phone use a different type of signal to construct the location estimate. If you are inside a building, you could take the phone outdoors so it has line of sight to GPS satellites. If you have WiFi disabled, you could turn WiFi networking on.
If the phone's reported location remains far away from where you are, you will probably still be able to find the place you want to check into by searching for it by name.