CC1101 not always working
The problem we have is that sometimes the signal is not received by a receiver as expected, this happens more as the distance is increased.

Yes. That's wireless communications for you: the further you go away, the less power you receive, the more multipath you might see, and thus, the worse your SNR gets.
Noise introduces errors. Deal with it!

The distances are not very long

&quot;not very long&quot; is err... not a specification? is &quot;not very long&quot; 10 cm? 100m? 10 km?

Any pointers on what the problem could be? Is this likely to be poor hardware design or incorrect firmware settings?

Well, something close to ground will not have a free Fresnel zone between a receiver and a transmitter, so little power can be received, even with good design and good antennas.
I'd bet on a combination of inappropriate design, antennas and this natural limitation. Also, you might not be picking a sufficiently robust mode.
My best guess is that if putting your transmitter up high solves your problem, you're simply in a situation where propagation conditions due to the close ground are too bad.