There are times where radio propagation is absolutely horrid, and even the best of equipment will get nothing but static. Back last month, we had a geomagnetic storm that just wrecked havoc on the HF bands. Unlike VHF frequencies used in the continental ATC centers, HF signals can vary greatly depending on the time of day, sunspot cycles, ect. In most cases, VHF tends to keep the same range of reception.
What Ken said. And we are going through one of those horrid radio propagation periods as I type this.
Also, the tuning schedule occasionally puts the receiver on a frequency with little use, and there are peak times of day and low points, so you have to listen more and you'll hear traffic.