[Lisa~]

Usually a barbell is considered more difficult/advanced for a single-leg movement. Since you hold the barbell on your shoulders, your center of gravity is much higher. That makes balancing the single-leg movement more challenging.

I hear Julie on the "work your grip" comments and agree to some extent. But for higher rep work you do sometimes get to grip failure in a most frustrating way. For step-ups you might be able to temporarily brace the DBs against your waist/hips for a couple of reps to rest your grip for a moment mid-way through the set. You might consider straps for the last set, but wait as long as you can before you start using them. Work your grip as long as you can.

But, like Jane suggested, it might just be that your step isn't high enough. You get that step-up high and you might not need the big DBs!