Heating Woes

[SvXSubaru92]

My dad drives our SVX to work in the morning, and when he first started driving the svx to work, the heating did not work to great. We disconnected the hoses to see if the coolant was getting to the core, I don't remember what we did, but I think we put some pressured air through it. The heating worked fine for about a month, but now its blowing cold again. What should I do? Thanks for help in advance.