Why Does My Milk Not Come Out When I Use A Breast Pump?

I had my son last Friday and I breast feed him
tand when I breast feed him my milk comes out perfectly fine but I tried using a breAst pump and nothing came out I had to have my son suck on my boob for a little then put the breast pump on right away and I got some like one sip but now nothing wil come out and it’s a hand pump btw anybody know how to help my problem or if this happened to you?

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