Squelch Settings
Blocked or "Stepped On" transmissions.
Other calls on "Guard" frequency.
Language differences
Different makes and models of TX and RX
Distance from TX, line of sight between antennas.
Bandwidth of equipment, The VHF AM band was previously split into 360 channels, then 720, and now 760 or 2280 8.33 khz channels.
Old equipment may still be in use in some GA aircraft or Gliders.