You are probably using a constant-time (CT-HNCA) version of the HNCA experiment. The maximum number of increments in the dimension involving constant time is limited to "ni * delta < CT", where ni is the maximum number of increments, delta is the increment in the indirect dimension of interest and CT is the constant time. delta is calculated based on the required spectral width in the indirect dimension.
Increasing the number of increments in the indirect dimension for 'standard' HNCA experiment does not always give higher resolution because the presence of unresolved J-coupling causes causes peaks to start 'splitting.' The CT-HNCA experiment was introduced to overcome some 'splitting' when the number of these problems.increments is increased in the indirect dimension.
