For folks who aren't understanding the new hero AI update: calling out targets (holding ctrl + clicking on an enemy) is much more important now
Your heroes will still go straight for your called target, but the difference is now if they get blocked, slowed, or stuck they won't just stand there and be useless - they'll attack the blocker/nearby enemies or attempt to re route until they can get back on track.
This is a huge improvement over the old system where anything in their way would prevent them from doing anything at all, but if you don't actively utilize the call out function you'll notice that they don't just automatically switch to your target as quickly.
If you're not on PC be sure to map target call out to an easily usable button.