It could be any number of things.
1. You could be in a 'dead' zone for the WiFi you are trying to access. Meaning the WiFi doesn't reach that area, or doesn't get good signal there.
2. You could be accessing a WiFi that doesn't have good connection at all.
3. The WiFi could have signal issues, either due to the modem/router or because of issues with the ISP.
4. There could be to many devices accessing that particular WiFi which can cause signal strength to drop. Especially if there is a limitation on the amount of devices that can connect to it.
5. Your data on your device could be interfering with the WiFi connection. It happens with some devices. To know for sure if that is it you would have to turn off data and see if the WiFi...