The incendiary bullets also didn't work, since they made small holes, there wasn't enough air in the hydrogen to cause it to ignite.
Then they started mixing exploding and incendiary rounds. After that they had some luck, but it still took concentrated fire to rip the bladders open, mix enough oxygen in and then ignite the mix.
The little observations balloons were easier (but still one of the most difficult tasks for a pilot due the the amount of defenses places around them)