Where are you located vs. where was the LZ? Terrain could have been between you and E-151's position. Remember, the chopper is approaching from above and thus most terrain is no-factor as far as degrading radio comms.
FERN is simplex so your often only going to get one side, especially the one in the air.
A good hint there is to listen to a repeatered FERN or Clear via the Aurora, Arvada, Westy, Lakewood, West Metro etc permanant patches. I find Aurora's to be the best for FERN across the entire front range and Arvada/Westy to be the best for CLEAR. But your mileage may vary.
Terrain must have been the problem. I was about 20 miles away from the LZ. What are all the talkgroups for the Repeated FERN and CLEAR channels? Are they only repeated in the Aurora and Westy areas?