Well I know it's tempting to just say you're having bad luck and write it off as that, but as I'm sure you're already aware of this possibility I'll try to think of a number of possibilities.

1. You're just unlucky:
Most likely answer and with an obvious solution, just keep trying. How ever it's also the least helpful and most annoying answer to be given as well as it's a possibility you're already aware of.

2. the chest item was never or improperly seeded to the dungeon as a drop:
In this case, you'll never see it til it's fixed.

3. non-equal probability: maybe the bard chest is meant to be a whole log rarer than the monk one? seems odd if they did that, but you never know.

4. Bizzarro bug: something akin to the "butterfly effect" ( a long chain of 1 in a million trivial, seeming unrelated events) has cause the RNG to work improperly, and this is the result.

5. Divine intervention:
Some being with power over this doesn't want you to have it. given that God doesn't fit under a microscope, I'd be at a loss as what to do in regards to this.