In a proposal for a website, you should never just throw a mock-up out there.
When creating designs for a client, it's pretty important to work with them and find out what they need. Any client who simply takes a mockup offered from proposals is usually skipping the entire decision process and hoping that they can get a free (or at least cheap) design that they like from freelancers offering mockups.
Try and look for clients who are requesting graphic designers with portfolios. Provide live, previous examples of work you've done (with previous clients' permission), and build a portfolio to sell yourself to a client.
Any work you do should be paid for, in general; in some cases offering value to a client can be a good thing (and will possibly help them to reconsider you for their next piece of work), but when a client is fishing blindly for freelancers and saying something along the lines of "Make me a design like x, and show me a mockup", and posting this on a bidding site, they are hoping that they can cut as many corners as possible.
If I were you, I'd stop sending a mockup in a bid and instead send a strong portfolio and a resume with references when bidding for these proposals. Act professional, and in your bid explain your experience and previous projects, and possibly how you handle the design process.