Jenniffer White ...
It sounds like you need more powdered sugar in it if it runs out of the bag. You might be overwhipping as well. If you overwhip your buttercream will heat up and be melty and runny.
It sounds like our recipes are similar; here are the ratios I use:
2 lbs unsalted butter, at room temperature
16 Cups powdered sugar (4 lbs)
1 TBL plus 1 tsp vanilla extract
1/2 tsp salt
1 Cup heavy cream
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