There are mainly two solutions: either your phase model is incorrect, or your diffraction model is wrong, e.g. the application of a variable slit is selected but not used. I would prefer the second option since a wrong structure model usually does not affect only the first peaks. If your strongest peak is not the first one (sorry for the possible misinterpretation) texture effects are possible as well. Also they affect more or less all intensities, but for the highest it is often most prominent.