What you describe is very common, and I believe it is due to tightness in the hips. A well-rounded yoga practice that includes lots of standing poses will naturally open the hips, enabling the student —in time—to perform this transition with much less effort.
In class, I often make light of the fact that students may need to use their hands to bring one foot forward. In fact, I actually include it as part of my instructions. It can be very embarrassing for some students, and helping them stay lighthearted is important.
Let me also add that “core strength” is an often-misused term. Core strength and abdominal strength are not the same. One could have very strong abdominals but have no sense of core or inner stability. If we think of abdominal strength and core energy as one, we risk missing the importance of a true center or core. We reduce yoga to exercise.
Let me emphasize that abdominal strength is useful and important, and I apologize if I have reacted strongly to your words. But I feel that the distinction is worth mentioning, as so many teachers use this term lightly.
Maty Ezraty has been teaching and practicing yoga since 1985, and she founded the Yoga Works schools in Santa Monica, California. Since the sale of the school in 2003, she has lived in Hawaii with her husband, Chuck Miller. Both senior Ashtanga teachers, they lead workshops, teacher trainings, and retreats worldwide. For more information, visit http://www.chuckandmaty.com.