Is this new plantings or well established Pampas?
You could use a fertilizer to get your plants growing this spring, though Pampas usually is a fast growing plant.
Have the plants received some rainfall or watering? After a long dry winter your plants may need a drink to start growing this spring.
If you have some well established plants, early spring can be a great time to give a good pruning. This can rejuvenate the plants.