Based on a discussion in the Amplitude Discord, we've determined that it is sometimes not clear at all why line of sight is blocked. As an example, in this screenshot there is no clear explanation as to why this line of sight is blocked:

After the battle, I was able to determine that one of the two hexes the unit is shooting through is actually a forest. However, on the battle map it is not clear at all that this is the case unless you hover over each intervening hex. Instead of simply having a tooltip saying line of sight is blocked, you could add something akin to this screenshot. This simple change would help us understand what is going on, and thus say "Oh, that's a forest" instead of "Why the heck can't I fire? BUUGGG!" ;)