Hi Welcome aboard.
Generally speaking any lens like the 70-300 Sigma that also does macro, also has more compromises build into it. Its not going to be a stellar performer, but you should still get good shots.
What is the minimum focus distance?
Also try sticking it in full auto with flash up and see if that helps, you may just need good shutter speed.
For macro shots prime lenses like the 60macro or 100 macro will be good as a portrait lens and good as a macro lens. My preference is for the 100 or 100+ as you can be back from the subject a bit.
Is it only macro photos that you have trouble with or with the Sigma in general. Have you tried using live view to focus, so see if that improves it.
Canon Powershot S95, Canon 7D, 15-85, Canon 85 1.8 USM, Sigma 30 1.4, 50mm 1.8, Canon 100 2.8L IS Macro, Canon 70-300L IS USM+Kenko 1.4 Pro 300DGX, Canon 430EX II and RS 4 Classic