How do I get a vital wittiness to tell the truth in court if they are to busy.

i have a very vital wittiness who can prove the truth on some court issues,that have been ruled against me in a court of law.she has told me for the last three weeks,that she is willing to tell the court the truth,but every time I call her she is to busy that day,and my entire case rest on her testimony.i don't want to make her mad,but I need her to tell the truth like she said she would,what do I need to do,i asked her to meet with me to just sign a sworn affiadavid,but as I said,she tells me not right now,because she to busy,what can I do legally?