IFort not vectorizing loop in specific cases

I have a big set of code with OMP4.0 directives (target, simd...)

In one module the compiler throws lot's of warnings about "loops not vectorized with simd" although it should.

I cut the code down to the bare minimum that still produces this behaviour:

   SUBROUTINE simdTest

      IMPLICIT NONE

      INTEGER ::  i, j, k, sr, tn,nzb,nzt,nxl,nxr,nys,nyn
      REAL    ::  s1, s2, s3, s4
      REAL, DIMENSION(:,:,:), ALLOCATABLE :: u,v,pt,rmask,sums_l
      REAL, DIMENSION(:,:), ALLOCATABLE :: usws,vsws,shf

      !$omp parallel do schedule(runtime) private(s1,s2,s3)
      DO  k = nzb, nzt+1
        !$omp simd collapse( 2 ) reduction( +: s1, s2, s3 )
        DO  i = nxl, nxr
           DO  j =  nys, nyn
              s1 = s1 + u(k,j,i)  * rmask(j,i,sr)
              s2 = s2 + v(k,j,i)  * rmask(j,i,sr)
              s3 = s3 + pt(k,j,i) * rmask(j,i,sr)
           ENDDO
        ENDDO
        sums_l(k,1,tn) = s1
        sums_l(k,2,tn) = s2
        sums_l(k,4,tn) = s3
      ENDDO

      !$omp parallel do reduction( +: s1, s2, s3, s4) schedule(runtime)
      DO  i = nxl, nxr
       DO  j =  nys, nyn
          s1 = s1 + usws(j,i) * rmask(j,i,sr)
          s2 = s2 + vsws(j,i) * rmask(j,i,sr)
          s3 = s3 + shf(j,i)  * rmask(j,i,sr)
          s4 = s4 + 0.0
       ENDDO
      ENDDO
      sums_l(nzb,12,tn) = s1
      sums_l(nzb,14,tn) = s2
      sums_l(nzb,16,tn) = s3

   END SUBROUTINE

If you compile this with "ifort -openmp -O2" it will warn about the first loop. If you remove literally anything (even from second loop) it will vectorize.

Message from vec-report is "subscript to complex".

Could you explain that? IMO not vectorizing the first loop would lead to significant performance loss.