So, unfortunately, if the marshmallows haven't set at the time you go to slice them, there isn't really anything you can do. This could be due to not using enough gelatine for the amount of liquid you're trying to set. More likely, it will be due to not whisking until stiff peaks before adding the sugar syrup and then not whisking long enough once this is added. When making marshmallows, it is far more difficult to over mix than under mix, so when in doubt, let your meringue or marshmallow continue whisking for an extra minute or two! Fortunately though, if you have some un-set marshmallows, this is essentially just homemade marshmallow fluff - you can still use it as a topping for other desserts and it will still taste great, though it won't be the airy texture you expect from a marshmallow.