Bows are some of the weakest range weapons in the game. As they should be compared to modern firearms. There are however some ways to make them more powerful than they currently are without simply increasing their base damage (although that might be warrented too).
Make the armour piece value of an arrow scale with the bow that they are fired from. The same way that the base damage of the bow scales with the arrow fired from it. This way a bodkin arrow from a high powered bow would actually have some anti-armour capability while if a bodkin was shoot from a less powerful bow than it wouldn´t do much against armour.
Implement poisoned arrows. These arrows already exist in real life and can be made with stone age tools. These poisons are usually made from various plants many of which can be found in New England. Substances from poisonous of toxic creatures have also been used and we have plenty of those in the game. Many of these poisons can be very deadly and fast acting especially if the player simply where to combined many of these at high concentrations to make some kind of hulk slaying super arrow. This would give bows a way to deal with very tough and even armoured targets given that you only have to do a little bit of damage for the poison to get in a enemies system and start doing its work.
Increase the damage multiplier on the proper arrows and maybe make more types of arrows. The low damage of bows makes sense for the makeshift types of arrows. The things that are little more than sharpend pieces of wood and bits of ceramic and rusted steel tied to the end of a small stick. If you used such arrows than you should expect it to take a dozen shoots to kill a zombie even if they are shoot from a very powerful bow.
Where it gets wierd however is when you look at the multipiers given to proper arrows. While makeshift arrows have a multipliers of 0.75 to 1.25 things that are proper arrows only have multipiers of 1 (bodkin) and 1.5 (broadhead). This leads to a situation where a makeshift wooden arrow which can use a nail as a arrowhead being more damaging than a bodkin arrow.
One way to fix this would be to increase the damage multipliers for bodkin and broadhead arrows to something like 2 to 2,5. Another way would be to implement more arrow catagories. Currently the bodkin and broadhead arrows are made from any type of steel the survivor has laying around. So you could make a new catagory of high-steel arrows that have these higher damage multipiers. Other things include making seperate bone and a high damage flint (VERY sharp) arrow catagories.