Blue Shield is notorious for not paying out enough of premiums for care. If you're having issues though, figure out who your PBM. (Pharmacy Benefits Manager) is that Blue Shield is contracting the pharmacy adjudication out to.
Basically, get in contact with them, try and get an override, or get them to explain why you are not covered. If it isn't in the formulary your employer opted into, you're hosed; but there is usually one way or another to get them to give in, they will just make you go through maximum hoops.
Ask for their clinical people if you can't get any traction. They will likely be the ones with hand on the knob for whether a drug is "worthy of coverage" or not.
Magic words: Override,coverage decision appeal, utilization management/formulary question.
They will tell you to check a website. Look up your thing in their formulary, if not there, abort. If in there, but high-tier, prepare for the prior-auth gauntlet.
I gave up on BCBS and have resigned to paying out of pocket, but you might get luckier than me. They will generally fight tooth and nail to be difficult, but just keep a cool head and keep escalating. It's been a while since my last butting of heads but I'm gearing up for round 2.
Also remember: You can pay out of pocket, but you'll be at the whims of that pharmacy. They'll generally help you work through coupons.