do-loop ignores if-statement in FORTRAN

i'm trying to use an if statement in a do loop which is supposed to generate prime numbers. For that i used modulo to sort out the numbers. After it found a prime number i want it to go a step further and add 1 so that the next prime number can be found and added to the array pzahl. My Problem is that the loop seems to ignore that it should go a step further with plauf after it found a prime number so that it just keeps going till infinity... I tried to rearrange the contents of the loop and if statement but it's just not working :/ Here the code:

PROGRAM Primzahlen

   IMPLICIT NONE

   INTEGER :: start, plauf, n, a
   INTEGER, ALLOCATABLE, DIMENSION(:) :: pzahlen !array into which the  prime numbers should be added
   INTEGER :: input
   INTEGER, DIMENSION(:), ALLOCATABLE :: alle 

   PRINT *, "How many prime numbers should be listed"
   READ (*,*) input
   ALLOCATE (pzahlen(input))
   pzahlen(1) = 1 
   start = 2
   plauf = 1

 loop1: DO 

   ALLOCATE(alle(start))

    loop2: DO n = 1,start
       alle(n)= MODULO(start,n)
    END DO loop2

   IF (minval(alle) /= 0) THEN  ! This is what it seems to ignore.
    plauf= plauf + 1 
    pzahlen(plauf) = start
    PRINT *, plauf
   END IF

  start = start + 1

 IF (plauf == eingabe) then
    EXIT
 END IF
 PRINT *,  alle
 DEALLOCATE(alle)

END DO loop1

PRINT *, "prime numbers:" , pzahlen(1:input)

END PROGRAM Primzahlen 

I use the gfortran Compiler and write it in emacs if that helps to know. Would be very thankful if somebody could give me a hint where my mistake is :)