The box is overdamped

Your box gives a Qtc below 0.5, which is below critical damping. The response has no peak at all, but it begins falling earlier than a better-matched box would, and the low bass is quieter than this driver and this volume could deliver.

Is this a problem?

Often not. Some designs aim here deliberately:

• An infinite-baffle or very large sealed enclosure naturally lands low
• Designs intended for substantial room gain use the room's rising response to fill in what the box gives up
• Some designers prefer the settling behavior, though below about 0.5 the audible return is very small

So treat this as information rather than a fault. The question is whether you chose it.

If you did not choose it

The box is larger than this driver needs, and you are paying for volume you are not using. Either use a smaller box, which raises Qtc and fc together and gets you a flatter response in less space, or use a driver with a higher Qts that suits the enclosure you have.

Beware one false economy: making the box smaller raises the cutoff frequency as well as the Q. You gain flatness, not depth. If depth is what you are after, the answer is a different driver or a vented alignment, not a different box size.