I use sand sealer on all work because for me it seems to maintain uniformity when staining. I actually do not think that the sand sealer does block repeated attempt in staining darker, but could be.
Now, most people who stain use stain that is mixed with polyurethane. Its that quick way of staining and sealing at one pass. I find that many people complain because the stain will not get darker; they are using this and the poly is building up a barrier to deter the second stain pass from getting darker. Could this be your problem?
If not, then try wood conditioner not sealer. This might help you. sand the door down lightly to break the sand sealer barrier and use conditioner and then try special walnut or dark walnut or honey oak mixed with red mahogony.
Good luck, hard to fix this over the net, but your plan of playing with stains with a sample is the right course.