Online exercise is enabled for our option plan, but certain holders can't exercise.

We turned on online exercise for our option plan a few months ago. Most holders can go through the flow without any issues. But we have a handful of early exercise grants where the holders are hitting a wall. The exercise page either shows 0 options available or won't let them proceed. The grants are outstanding, the plan shows exercise enabled, and we can't figure out what's different about these specific grants. Any ideas on what to do?