DIAGNOSIS: Yeah, this is common. It's usually one of three things: missing Trueplay tuning, incorrect TV audio format, or physical placement blocking the upward-firing Atmos speakers.
FIX: First, run Trueplay tuning if you haven't. You *must* use an iOS device (iPhone or iPad); Android can't do it. It's in the Sonos app under Settings > System > [Your Arc] > Trueplay. Wave your phone around like it tells you to—it makes a massive difference by calibrating for your room's acoustics.
IF THAT DIDN'T WORK: Check what's above the Arc. If it's mounted directly under a TV that overhangs, or there's a shelf less than a foot above it, the upward-firing drivers are bouncing sound right into an obstacle. You need at least a foot of clear, reflective space above it for the height effects to work.
HEADS UP: If your TV is sending the Arc a stereo (PCM) signal instead of Dolby Digital 5.1 or Atmos, it will sound thin. Go into your TV's audio output settings and switch from 'PCM' to 'Auto' or 'Bitstream'. The Arc's display should briefly show 'Dolby Atmos' or 'DD 5.1' when content starts.