There is not enough information to properly answer the question...
1) Are you sure that you had it donned correctly? Is there a possibility that you had undergarment/hair in the neck seal area? Was the neck seal laying flat and even around your neck or folded over properly if it was neoprene?
2) Do you have a thin neck such that when you grimace or swallow, your throat or tendons stand out from the neck and possibly break the seal?
3) Is your neck seal properly fitted? Is it trimmed too large?
The majority of the time there are leaks in the seal area, unless there is physical damage, it is usually from donning procedures.
Were you taking a drysuit class when you took the drysuit into the pool? Your instructor should have been able to help, or at least let you know what kind of seals you have. If not a class, then I reccomend that you either take a class, or get some oversight/help from a friend who has used a drysuit before.