Question: How exactly were Dracula's servants successful in stopping Van Helsing in 1897? What did the monster hunters do wrong?
Answer: They caused enough chaos and terror in the hunters that the girl couldn't focus on reading the incantation. Van Helsing was desperately trying to get her to hurry up and finish but she just couldn't get it done in time with everything going on around her. Apparently Van Helsing's group was not prepared for the undead bursting from the ground and frightening everyone.
Answer: The girl finished the incantation and opened Limbo but it only took some of the hunters, the girl and Van Helsing; it didn't take Dracula so it was a failure of a mission.