Even the best small deep freezer can potentially fail at keeping your items frozen, particularly when they're placed in a hot environment. Get a digital thermometer if you're worried about this, and turn down the temperature controls (if they are featured on your unit) to the lowest setting possible.
You'll find that your food stays cool this way when the outside temps getting warm. If that doesn't work, it might be a good idea to let a little frost build up in the freezer, if there's enough room inside for you to do this.