Mapping values to title blocks using Autolisp routines

I am trying to map values to my title block using an autolisp routine. I have entered (getenv "NEXTSHEET") & (getenv "PREVSHEET") into the descriptions fields and mapped these routines to 'NEXT_ATTRNAME' & 'PREV_ATTRNAME' attributes. When I run the title block updates no values appear on my drawing. Please note that I have included the program in my start-up suite. Please also note that when I enter 'NEXTSHEET' or 'PREVSHEET' on the command line, the routines are called and the correct values are shown on the command line. I just need these values to be visible on my drawing. I have attached the Autolisp routine that I am running. Can some please advise what I am doing wrong.

Thanks in advance.

Nextsheet.lsp

An alternative approach will locate all title blocks in dwg

; changes to issued for construction
: thanks to lee mac for original code
(vl-load-com)
; 1.  Get current date in mm/dd/yy format.
(defun ddmmyy (/ x today)
    (setvar "cmdecho" 0)
    (setq x (getvar "CDATE"))                 ; get current date
    (setq today ( rtos x 2 4))                    ; convert to a string
    (setq date (strcat (substr today 7 2) "."    (substr today 5 2) "." (substr today 3 2) ))
)

(setq oldtag1 "DRAWING_STATUS") ;attribute tag name
(setq newstr1 "ISSUED FOR CONSTRUCTION")
(setq oldtag2 "REV_NO")  ;attribute tag name
(setq newstr2 "0")
(setq ss1 (ssget "x"  '((0 . "INSERT") (2 . "DA1DRTXT"))))
(setq inc (sslength ss1))
(repeat inc      
(foreach att (vlax-invoke (vlax-ename->vla-object (ssname SS1 (setq inc (1- inc)) )) 'getattributes)
(if (= oldtag1 (strcase (vla-get-tagstring att)))
(vla-put-textstring att newstr1) 
) ; end if
(if (= oldtag2 (strcase (vla-get-tagstring att)))
(vla-put-textstring att newstr2) 
) ; end if
) ; end for
) ;end repeat
(setq oldtag1 "REV-NO")
(setq newstr1 "0")
(ddmmyy)
(setq oldtag2 "DATE")
(setq newstr2 date)
(setq oldtag3 "AMENDMENT")
(setq newstr3 "ISSUED FOR CONSTRUCTION")
(setq ss2 (ssget "x"  '((0 . "INSERT") (2 . "REVTABLE"))))
(setq inc (sslength ss2))
(repeat inc
(foreach att (vlax-invoke (vlax-ename->vla-object (ssname ss2 (setq inc (1- inc)))) 'getattributes)
(if (= oldtag1 (strcase (vla-get-tagstring att)))
(vla-put-textstring att newstr1) 
)
(if (= oldtag2 (strcase (vla-get-tagstring att)))
(vla-put-textstring att newstr2) 
)
(if (= oldtag3 (strcase (vla-get-tagstring att)))
(vla-put-textstring att newstr3) 
)
)
)
(setq ss1 nil)
(setq ss2 nil)
(princ)

Thanks for your reply. However I am unclear on what this code is doing exactly. I am trying to set-up my template to contain attributes that detail the previous & next sheet number of the drawing set. These attributes are populated and visible when I update these fields with constants. However are not visible when I try to run the attached routine and update these attributes with variables.

Please advise.

Thank-you.

I think I am heading down a different path.