The best one I can think of that more or less exactly fits your description is "The Unit" by Sexually Broken. Obviously the main focus of this movie (and website) is on bondage and sex, but this video does contain one of the best quicksand scenes I've ever seen - it's simply awesome!
She does indeed get stuck and when they find her she begs for help and they respond with "We'll help you when your.. uh.. under!". They also tease her by picking up a tree branch and holding it just out of her reach!
The only thing that's missing from this scene in my opinion is a full submergence, she clearly tried to go all the way under but it was too thick. She got everything under bar the very top of her head though, eyes, nose, and ears all went under!
There is a trailer and some screenshots here:
https://www.sexuallybroken.com/sb/play.php?id=JC0ODBT97FW1QNP25LHV There are brief shots of the quicksand scene in the second half of the trailer.