Most people cut back on fertilizing in the summer months, but it is possible to sustain growth during the summer, just more hazardous if things aren't managed carefully.

Fertilizer is not the only answer. Plants in pots depend on us to keep nutrients available. Most potting mediums have very little, so regular applications of water soluble fertilizers are necessary for sustained growth. A plant makes its own food, through photosynthesis, so adequate sunlight is necessary for growth. Water is also essential to photosynthesis, so regular watering is necessary.

If the plants are suffering from a lack of nutrients, then watering with a weak solution of a balanced fertilizer might help some. For significant growth, if you can keep the root zones cool (and I can't stress how important this is) increase the amount of sunlight the plants are getting and apply a balanced fertilizer regularly along with regular watering. You should see increased growth. Be careful, take small steps and keep things balanced. Big increases of any of the above may lead to crown rot.
Ed