If the shampoo bar isn't lathering well it's more likely hair can remain coated with the soap and more difficult to rinse out thus leaving residue. If this is the case you may try building a lather in a loofah or your hands first instead of just swiping the bar across your hair.
When you first switch from a liquid shampoo to a natural shampoo bar, you may experience a transitional phase where the hair appears more greasy and dull. During the transitional phase it's also not uncommon to have more challenges with residue being left in hair because natural shampoo bars don't strip hair in the same manner that conventional sulfate/detergent shampoos do. It can take some time to remove buildup from previous hair products as well as for scalp to balance its oil production.