realtor refuses to set up lockbox

I have a seller who ended up using a realtor through our transaction. She has done nothing but slow down the process and now that our contract is signed, she refuses to let me set up a lockbox and insists on having the only key and letting me in the property each time I need access during my due diligence.

To me it seems like she is impeding my expectation of reasonable access. What can I say to her do get her to stop breathing down my neck?  She has nearly killed this deal 3 times!