As a manager, I email people requesting information within a certain
time frame (usually one to two weeks). However, they often forget to
acknowledge or reply in a timely way. What are some tactful and polite
responses that I could progressively send to the recipient showing the
importance and urgency of the original request?

skooter...

I would simply repeat the email, flagging it as high priority
and requesting a read receipt.

THere is a couple of ways.
1. Resend the request adding the aformentioned flagging and high
priorty markers to the email. Additionaly, add some reasons why it is
important and how it can help them.

2. Resend the request, call the receipient and clarify any
misunderstandings. If the receipent still does not respond, report it
to their supervisor or counsel them for non-compliance if the
responders are within your supervision.