Some blenders produce heat when blending on max speed or for longer periods, and this heat is transferred into the drink during blending. If your blender does this, it can cause the ice to melt quickly. To remedy the situation, you can add some ice and chilled water to the pitcher for a few minutes to chill it before you make your slushie.
Alternatively, when making your drink, reduce the amount of liquid and increase the amount of ice/ frozen fruit you add. Make sure all your ingredients are chilled or frozen, try not to add any room temperature liquids or fruits. And always try to blend for the minimum possible time, then pour out the drink as soon as it’s fully blended.