A WAH employer is looking for different things on cover letters and resumes then those in many brick and mortar companies. If you're not sending a cover letter with your resume then this might be one of the reasons you're not getting hired. The cover letter is your VERY first impression. Take the time to show you're very serious about the job they have open. Tailor your cover letter to that specific position.
Overall, you want to pay attention to detail, outline why you're the best person for the job, and point out that you have what is required for the job. And most importantly, run spell check and grammar check. The world's most eloquent cover letter is negated if you misspell simple words on it and then brag about your "attention to detail" in it.