ihernandez -
Sure - there are a number of reasons.....
Bad cable(s) or connections at the card(s).
Bad cable connection from the card to the antenna (bad cable-loose ground, water intrusion, etc).
Antenna damage.
LoS issues, such as 'knife edge defraction', or an obstacle closer to one antenna than the other, also there can be reflections making the signal out of phase with the main signal as they both arrive at the antenna, proxmity of other RF noise inducers (lowering the effective S/N ratio).
R/
Thank you Thom for your reply, I will start by changing the cable. When I first installed this link, it was flawless but with time one side degraded too much.