4/5 year old shih tzu and 4 month old beagle/husky mix don't get along?

I have two dogs that don't get along very well. One is a 4 maybe 5 year old female shih tzu (got it at the animal shelter and i don't know what age it is) and one is a male 4 month old beagle/husky mix. Like i said, they don't get along very well and my family and i don't understand whether it is the age difference or maybe gender difference, but i don't know. What do you think it is?