Short Answer:
In most cases, the issue is not transmitter power or antenna size, but local noise or receive conditions at your station.
Why It Works:
Amateur radio communication depends on signal-to-noise ratio. If background noise at your location is high, weak signals may be present but completely masked, making them impossible to hear even though others can copy them.
Important Limitation:
Propagation conditions and antenna differences also matter, but they are often secondary to local noise issues.
Elmer Tip:
If stations seem to disappear when you reduce receiver bandwidth or turn off nearby electronics, noise is likely the primary limiting factor.
Next Step:
Compare noise levels on each band and identify possible local noise sources before changing antennas or power.