The transmit power of an AP changes based on its data rate. In other words, lower data rates have a longer range. I've noticed this behavior myself, and I'm not the only one.
It sounds like you're either on the fringe of your AP's range and/or your device is doing a poor job negotiating a connection speed. For the former, move closer to the AP or use better or additional APs. For the later, update the firmware of the APs and update the WLAN drivers for the devices.