If you are making your own gelatin zymogram gels, make sure to completely dissolve gelatin at about 60C, cool it down, and mix well with other reagents. When you run a zymogram gel, don't add the reducing agent to your samples and don't boil them.

Good luck!

When preparing the running gel solution, add gelatin stock solution (10 mg/ml in destilled H2O) to get a final gelatin concentration of 0.1% (1 mg/ml). 

Furthermore, you can increase the incubation time (overnight, 24h, 48h) in order to observe larger activity bands.