[cem] missing strata for CEM matched observations

Hello, Does anyone know why I may be getting missing strata for CEM matched observations? These matched observations do have cem weights, however they do not have a strata number. I checked the variables that I match on and they are all numeric and not missing. So it is a little bit puzzling to me why strata values are not generated for a huge subset of CEM matched observations. I would appreciate any help on this. Additionally, does anyone know if CEM uses matching with replacement or without replacement? Thanks for your time. Best, Hulya