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Thanks @bscuderi! I was having this exact same problem. The Lambda test was passing and I could see all my entities, but Alexa discovery failed to find anything. I excluded all my Denon devices from Alexa in my configuration.yaml (below), restarted Home Assistant, and BAM! It started working after the 2nd discovery attempt.
alexa:
smart_home:
filter:
exclude_entities:
- media_player.denon_avr
- media_player.denon_avr_2
- media_player.denon_avr_s940h
- switch.denon_dsw_1h
- media_player.living_room
- media_player.living_room_zone2
- media_player.bedroom
- media_player.living_room_2
- media_player.master_bedroom
- media_player.master_bedroom_2
- media_player.master_bedroom_heos
- media_player.theater
- media_player.theater_2
- media_player.theater_3
- media_player.theater_heos
- media_player.theater_zone2