One of the worst things you can ever say as a developer/programmer is "It's working for me." Unless of course that is immediately followed by "..but let's dig in and figure out why it isn't working for you." This type of thinking is a cancer within development teams and certainly isn't customer focused. Software breaks, apps don't work properly, and if you're a developer it's your responsibility to do whatever it takes to resolve your customer issues. A few years ago one of my dev teams was having tremendous difficulty replicating a customer-reported issue. One of the developers started the whole "It's working for me" thing and my manager heard it. He jumped in and quickly asked my developer if he was going to package up his computer and ship it to the customer. What a great response.
When you're up against tough issues, try not to think about how hard or how long it might take you to resolve the issue, but resolve to fix it no matter what it takes. Then, and as early as possible, assure your customer that you will do whatever it takes.