Spectating the Battlegrounds on a friend, they selected the Fishy sticker. and has plenty of deathrattle minions on the board and the fish only appears after the first deathrattle has activated, they have 2 spaces on the board so it isnt a space issue. I think unless I’m missing something the stickyfish might need to be jumped up in the activation order so its there at the start of combat as it suggests rather than “appears after your first deathrattle minion dies” 
Cheers people
Edit, think I spotted something I’m missing. (feels stoopid) My friend is playing Illadin, so left and right minions attack, if the fish was out it would always appear on the right so would die. This appears to be working as intended but instead of deleting this I’m leaving it here in case it helps someone
toodles