This might seem a bit obvious, and I may have misunderstood your question, but make sure that Yoshimo is selected when you are at the stealing screen. What I do is to select my thief (Jan) and get him to initiate a conversation with the merchant.
Use a potion of master thievery if you have one. Apart fromthis I can't think of anything else.
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"Butt-kicking for goodness!" - Minsc
"Cities always teem with evil and decay. Let's give it a good shake and SEE WHAT FALLS OUT!!" - Minsc
