What i understand is you are selecting the keyframes and then choosing Stretch Section option? If that is the case you need to select Stretch Selected Key Frames option and it will pop up a new text bar to enter New Time. It should show you that text bar nonetheless though so maybe it is showing up in another monitor if you are working with multiple screens, or you may just be missing it since it is black and gray and likely to blend in with Matinee UI. Let us know how it goes and make sure to replicate and upload a sample project if it still doesnt work so that we can turn this into a bug report for Epic to test.