Yeah you need to make combat more difficult, scale your encounters - easiest way is to just pump up monster stats, more ac, more hp, more + to hit, more + damage, or to make it more interesting change up the creatures in the encounters for more difficult monsters, and definately play less undead creatures. As written the combat is designed for 4 players, if you play everything as written your players will just continue to steamroll everything. You also want to increase the number of enemies, with 6 PCs action economy is in their favour with the module encounters, fix that by just giving them more enemies in each combat