First, is this plant potted or PIG? If the former, when was the last time you repotted? Fertilizer is not always the solution nor is lack of fertilizer always the issue.

Potted, purchased this year....then what would be the issue if it's nit fertilizer?

Could be a variety of issues

http://www.hiddenvalleyhibiscus.com/care/yellowleaves.htm

For potted plants:

If you're using a "bloom booster" fertilizer, I'd start there as a potential cause. Of the NPK %s, phosphorous (the middle number) should always be the lowest. Fertilizer RATIOS are different than NPK %s. for example: 20-20-20 is a 1:1:1 ratio fertilizer while 24-8-16 and 9-3-6 are 3:1:2 ratios. Hibiscus like their NPK %s to be somewhere close to 3:1:4 in ratio. Since I use Foliage-Pro 9-3-6 for almost everything else I grow (a 3:1:2 ratio), all I need to do to change the ratio is add a little potash to the fertilizer solution: or better yet (this is what I actually do), add a little Pro-TeKt 0-0-3 to the fertilizing regimen. The 0-0-3 also contains silicon in opalene form which strengthens cell walls, helping to guard against insects, disease, extremes in temperatures and moisture levels.

Tight roots cause shedding of foliage and the yellowing of leaves that precede the actual abscission. Hibiscus have such vigorous root systems that they really should be repotted every spring. Repotting is different than potting up as it includes bare-rooting and removing problem roots + a good fraction of large roots that serve little purpose other than plumbing. Repotting completely eliminates the limitations imposed by root congestion for a time, while potting up ensures that the limitations will be a permanent limitation and part o the plant's existence until it's corrected by someone's hands in the root mass.

Over-watering/soils that drain poorly cause a drought response, part of which is the shedding of foliage and the same yellowing of leaves mentioned above that precede the actual abscission.

Sap-sucking insects are a potential cause.

Al