Slow drain in the bathroom sink that keeps coming back, what's the actual fix?

The bathroom sink has been draining slowly for a couple of months and I've tried the usual fixes, plunger, drain snake, baking soda and vinegar, but it keeps coming back within a week or two. I'm starting to think there's a blockage further along that I can't reach, or maybe something wrong with the trap or the vent. I came across Kitchen and Bath Palo Alto plumbing services while looking into whether this needs a professional, but I'd rather hear from people who've dealt with a recurring slow drain. Did it turn out to be something simple like a blocked vent, or did it need proper rodding? Also, how do you tell when it's genuinely a plumber job rather than something you can keep attacking yourself? I'd rather pay once and have it sorted than keep fighting it.