Problems with Siemens RCR10/433 Receiver?

I had a Boss therm which had to be replaced as the receiver was not sending the signal to the boiler despite the green and orange lights being on, when knocked it eventually worked. The engineer replaced with a Siemens unit above. The receiver however kept getting intermittent red and green lights, the engineer came back and reset the units, this still failed. The engineer had to come again and replaced the units for a second time. Now I am getting the red light again on my second unit, intermittent with the green light.Please can someone let me know what they think is happening? Could there be a problem with the boiler, or is it these units, do I need a third unit, or will that go the same way!