U can’t unlock it... developer is aware of this issue ... but there is no priority currently to develop this last 3 companion... it may come in the future but not so soon...
“When Allen gets his balloon, the player will get rewards for each level you complete and use them at the beginning. If you lose or quit a level, Allen will lost almost all balloons, just one remains.”
This is very bad. first try restart ur device and update to latest version to see whether it work or not. else u may need to uninstall and install... but do take note the extra life and booster will be reset ...