Question on simple sit

Little Miss Roux got to accompany us on a trip to Home Depot this morning.  We were shopping for a new washer and dryer, so lots of opportunity for her to practice her simple sits while we perused various machines. 
 
My question is this: there were several occasions when she just wasn't connecting the upward leash pressure with putting her butt on the ground, even when I waited her out.  I think its a combo of things: very exciting new place with lots going on, plus I only had her on the martingale.  Is it okay to step into her to get the sit in this situation?
 
Thinking about it while writing it, I'm going to guess no, you want just the pressure to be the cue?  I suspect the prong would have helped her in this situation.