Was there a reason to change?
A buyer who has not felt the cost of the problem has very little reason to prioritise your document.
Did we understand the decision?
Many proposals are sent to people who like the solution but cannot buy it alone. A useful sales conversation maps the decision before the offer.
Did the proposal explain an outcome?
Specifications describe what you provide. A decision becomes easier when the buyer can see what changes after implementation.
Follow-up is still necessary
But it should continue an agreed decision process rather than repeatedly asking whether somebody had time to read the attachment.
Silence after a proposal often begins before the proposal is written.