Bots will only be able to access messages/events which mention the bot via @botname, unless the bot is in a 1:1 space, in which case it is not necessary to mention. If that mechanism doesn't seem to be involved, then you might look at the bot code to see if it is filtering incoming message events - i.e. to only your user id.
Not certain, but it may be possible to cause something like that if specific types of limitations/restrictions are configured in the space re external/non-org users. If it works in a non-restricted space (e.g. if you use a free Webex account to test), then you might check into those settings with your org admin.
