My suggestion is, don’t overthink it.
If you’re talking through Instagram or another platform it happens to me too honestly 
Sometimes I’ve got so many things to do that I forget to reply to my DMs.
Moreover, if you’re talking via episode accounts, it could be that they have their normal profile as the first account so they don’t see your notification (or vice versa)
But don’t stress over it 
if they keep talking to you as nothing happened, probably it’s because they just didn’t have time to reply and so, they just go on with the conversation 