This may have been caused by either:
1. Too much water being added, leading to the paint becoming soft and lumpy
2. Storing the paint in conditions which are too hot
To rectify this, try:
1. Drying off your paints with a clean kitchen towel. Doing so should absorb most of the moisture. Use a dabbing motion so as to reduce the amount of paint wasted.
2. Storing the paint in a very cool room. Doing so usually sets the paint hard again. If you face paint in hot weather you may want to invest in a small cool bag which you can store your paints in during the day to stop them from getting too hot.