I did umbrella-sampling simulations on a 45 residue protein (non-symmetric shape) translocation through lipid membrane. After that, I calculated the histogram for different bins and found one small gap at a specific window. Checking the z-coordinates of the protein center of mass at that window, I can see that the protein always prefers to stay at a lower position than the target height. Even after increasing the force constant, the same phenomena happened. I am wondering if that is a problem, and if yes, how to solve it.