Bubbers, I'm not a pilot but I know radio. Finding the source of a VHF or UHF signal in mountainous terrain without at least a tuned directional antenna can be a tricky thing, as signals this high in frequency will reflect off mountain slopes and any relatively flat surface. If that surface is located and shaped correctly it may even provide a stronger received signal than the original source should both be visible to the receiving antenna, giving you a false bearing. Which brings me to another point- if they were deep in a valley that had peaks in every direction around them, you might have to be almost directly on top of them to receive anything as terrain also absorbs and weakens any signal which it doesn't reflect- something lost and something possibly gained together.
Then there is also the searching aircraft and it's antennae to consider. An old friend who Captained the local CAP in the mountains here told me how he made some rookies turn green as he banked the plane heavily to each side in turn so as to shield the bi-directional DF loop on top with the metal skin of the airframe so he'd know whether to head left or right. He was another Ham operator like me so he understood what he was up to even though it wasn't in the CAP manual. A regular omnidirectional antenna on top of any metal-clad A/C would be shielded similarly, but without any assurance of direction otherwise, and possibly with some signal still reflected by the tail empennage in back lessening this effect. Most S&R units now use a rapidly switched doppler array that puts you on the strongest-signal quadrant in well under a second once it has the signal- amazing how far we've come in 20 years time. But you have to capture the signal no matter the technique and equipment used, and that can be the biggest problem
ELT's are low-power transmitters, so anything that degrades the signal will have a very marked effect on the range it can be detected at. If it's antenna got pushed underground just below the surface or equally covered with dirt, half of it's signal strength could be lost- lots more if it were covered with metal from the hull as well. Then there's signal orientation loss. Radio waves propagate in a planar fashion, and the further you are away from parallel antenna polarization on both ends the higher this loss. IIRC, aircraft use vertical polarization. This loss can be as much as 20dB, which reduces signal strength by about a factor of 8. Double that with a buried ELT and not now have as little as 6mW effective transmitting power. Foliage reduces signals in this frequency range too, so if among firs or pines there could be maybe another 25% of the signal lost. And there could be weak batteries when activated and time reducing that. There is more RF power available from a spark made by shorting a regular 9V transistor battery with a foot-long wire than this.
It is indeed sometimes like looking for a needle in a haystack, but this 'needle' runs on batteries which will make it disappear soon when they're spent. I hate to say it, but if they haven't located the signal by now it's not likely to happen.