"Waiting for sender" means that either:

• the sender has not completed uploading the Package, or
• the Package has expired from Pando's storage proxies (servers) and neither the sender nor any other recipients of that Package are available to transfer it to you.

Often the transfer will resume on its own when the sender becomes available (comes online). Sometimes clicking the "pause/resume" button will jumpstart a stuck transfer, but your best bet is to contact the sender and ask him or her to resend or forward the Package to you.