IMHO, chemical drain cleaners are not worth the cost. Undo the drain pipes below the sink and clean them out. If the blockage is further down try a stiff piece of thick wire with the end rolled over, or try a hosepipe. Just make sure whatever is on the end of the hosepipe or the wire doesn't have sharp edges that can get stuck somewhere deep in the drain lines. I know this sounds messy and it is, but that to me is the only sure way of unclogging drain lines.
Thanks for the replies folks. Ended up having to call a plumber. Kept running a "snake" down the line but to no avail. I'm goning to eventually re-plumb this line as there are some "hokey" plumbing errors that I see. Again, thanks for the replies and I won't use "drain cleaner" chemicals since it appears nobody has had any success with them.