Are they stock spec/size bulbs? Are you getting any errors/lights on the dash?

 

Swap the bulbs side to side. If the issue follows the bulb, it's the bulb. If the issue stays on the same side, you have a ballast that needs to be replaced.

 

^ what Zmon said.

What brand are the D4S bulbs? D4S bulbs are notorious for having finicky lifespans and color temperatures. Most major/reputable brands last a couple of years on average and have as-advertised/-listed color temps. Most unbranded/generic bulbs have lifespans as short as a few months, and color temps may visibly vary over time.

Could be a case of "you get what you pay for".