It's tempting to tell your client you will take care of everything, but it's not very realistic. Your client is generally your primary source of information, so have her prepare a timeline, gather records, and make a list of witnesses. The more familiar your client is with what is going on in her case, the better her understanding if you have to give bad news. Clients need to be informed of problems in their cases. Ideally, this should happen in the initial client interview, if you're aware of the problem at that time.
Clients hate surprises.
Be realistic about the nature of the problem and your ability to resolve it. Don't lead your client to believe an issue is insignificant when it's very important. The client then assumes it will be resolved promptly and have little impact on the case. Use nonthreatening neutral language. Don't become defensive with your client.