This answer record explains the features that are currently not supported by Vivado Synthesis for DSP48 inference.
Solution
The following features are not supported by Vivado Synthesis during DSP48 inference:
SIMD mode that allows to do multiple parallel operations to be inferred into one DSP48 slice.
incremental count values greater than 1 such as 2,3,4 to be inferred into a DSP48 primitive. Currently incremental count by 1 does get inferred into a DSP48 primitive.