Your normal lens is either a f1.4, f1.8 or f2.0. The largest opening on your normal lens is almost twice the size as the largest on the zoom lens. The zoom lens will only let in half the light of the normal lens. That is why the normal lens will focus in a low light situation that the zoom lens cannot handle in Auto mode.
The solution to this is either more light on the subject or more practically, the use of the P mode which will allow the camera to set the ISO sensitivity (Pages 62 - 63 of your manual) to a higher range. This increases the image sensor's sensitivity to the light received.
Remember, the manual indicates that below certain light levels, manual focus should be used. (See pages 113 - 116 for details)
Just in case you do not have the manual handy, you can download a PDF copy at the link below.
Please keep in mind that my diagnosis & solutions provided are directly dependent on the accuracy of your description of the problem. As with any "do it yourself" fixes, success is a "team effort", since I can't see or touch the camera, and relies on the customer's manual dexterity and ability to follow the instructions well.
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