There's a few things you can do to get the best signal possible

1. Sensor distance from the ANT+ USB stick. Try moving the ANT+ stick closer to your bike and sensors. If this resolves it but that puts your computer too close to your bike, you could try a USB extension cable to get the sensor closer.  Here's a great USB extension cable that we use.  It's a beast at 16 feet
2. Sensor battery. We’ve noticed that when a device battery gets low, the reception distance decreases. Try replacing the device battery. Some devices report their battery condition over ANT+ and we display that on the Devices page. But it is not a universal feature and speed/candence sensors can’t report it due to a protocol limitation.Garmin GSC-10 and many heart rate monitors use these batteries
3. Interference. We've had reports of other wireless devices like routers interfering with the ANT+ signal. ANT+ is on the 2.4 GHz band.  If you have any wireless mice by your stick move them away or turn then off. Unplug any other wireless USB sticks you have plugged into the computer. We've also had reports of poor signal quality while time machine is running to a Time Capsule on a mac.
4. Obstructions. Try removing any physical obstructions between the sensor and the ANT+ device.
5. Magnet and sensor position. Sometimes the magnet or sensor moves out of position for your speed or cadence sensor. Make sure they are lined up.