What my vet told me is that when switching brands you must do so gradually mixing in the brand you are switching to (Purnina, you said.) So start off with a bowl of 90% Euk and 10% Purina and she how s/he does with that. Each day add a little more Purina and a little less Euk. This worked for me when I switched dog food brands but there is the chance that Purina just won't agree with your maltese. In that case you would want to find him/her another dog food altogether.
Hope that helps!