If a patient does not respond to citalopram, is there any chance that they will respond to escitalopram?

Yes, clinical studies indicate that escitalopram 10–20 mg offers superior efficacy compared to citalopram 20–40 mg, demonstrating significantly better responder and remitter rates (ref.1), (ref.2), (ref.3), (ref.4) and (ref.5).

References:

1. Gorman et al, 2002

2. Lepola et al, 2003

3. Colonna et al, 2002

4. H. Lundbeck, data on file

5. Burke et al, 2002