I had some problems with signal strength (= no connection anymore) after ~ 1 year.
Removing the cover increased the signal significantly, something must have happened to the plastic .. anyway.
After replacing the internal antenna with an external antenna(433 MHz < $10 at amazon http://www.amazon.com/3Meter-433Mhz-Antenna-Magnetic-Connector/dp/B00S0JQZIY/ref=sr_1_8?s=electronics&ie=UTF8&qid=1446923235&sr=1-8&keywords=433+mhz+antenna) the signal came back and is now at 7 (10 meters away). The max reading I got was 8 very close to the receiver
Make sure that the antenna is upright (or in the same direction as the transmitter antenna), if it is 90degrees off you will minimize the signal.
Attached 2 pics of (a)the internal connection and (b)how it looks like fully assembled
Karl