How to combine science images correctly?

[Aldocthor]

I'm trying to combine science images (not calibration) with IRAF imcombine task , to then perform photometry with the phot task. The issue is that when I do the photometry I get the same mag and mag_err values as the last image of the science files used to combine. So basically what is happening is that the combine file gets the same pixel values of the last file of the combine list.

So what I want to know is if there are other arguments, or tasks that I could be missing or if there is some theory about science image combine.

This are the parameters I'm using for imcombine:

      input = "@rot.lis"      List of images to combine
       output = "comb3"         List of output images
     (headers = "")             List of header files (optional)
     (bpmasks = "")             List of bad pixel masks (optional)
    (rejmasks = "")             List of rejection masks (optional)
   (nrejmasks = "")             List of number rejected masks (optional)
    (expmasks = "")             List of exposure masks (optional)
      (sigmas = "")             List of sigma images (optional)
       (imcmb = "$I")           Keyword for IMCMB keywords
     (logfile = "STDOUT")       Log file

     (combine = "median")       Type of combine operation
      (reject = "crreject")     Type of rejection
     (project = no)             Project highest dimension of input images?
     (outtype = "real")         Output image pixel datatype
   (outlimits = "")             Output limits (x1 x2 y1 y2 ...)
     (offsets = "none")         Input image offsets
    (masktype = "none")         Mask type
   (maskvalue = "0")            Mask value
       (blank = 0.0)            Value if there are no pixels

       (scale = "none")         Image scaling
        (zero = "none")         Image zero point offset
      (weight = "none")         Image weights
     (statsec = "")             Image section for computing statistics
     (expname = "EXPTIME")      Image header exposure time keyword

  (lthreshold = INDEF)          Lower threshold
  (hthreshold = INDEF)          Upper threshold
        (nlow = 1)              minmax: Number of low pixels to reject
       (nhigh = 1)              minmax: Number of high pixels to reject
       (nkeep = 1)              Minimum to keep (pos) or maximum to reject (neg)
       (mclip = yes)            Use median in sigma clipping algorithms?
      (lsigma = 3.0)            Lower sigma clipping factor
      (hsigma = 3.0)            Upper sigma clipping factor
     (rdnoise = "0.")           ccdclip: CCD readout noise (electrons)
        (gain = "1.")           ccdclip: CCD gain (electrons/DN)
      (snoise = "0.")           ccdclip: Sensitivity noise (fraction)
    (sigscale = 0.1)            Tolerance for sigma clipping scaling corrections
       (pclip = -0.5)           pclip: Percentile clipping parameter
        (grow = 0.0)            Radius (pixels) for neighbor rejection
        (mode = "al")