GAMS code results gives all the variables as zero

Hello! I have a homework minimization problem i am working on using the modeling program GAMS. I am stuck and cant seem to find the problem. The question is this:

My code is as follows:


set t time period /Jan, Feb, March, April, May, June, July,Aug,Sept,Oct,Novem,Decem/;

MC Material cost /20000/
HC Inventory holding cost /3000/
LH Labor hours required /212.5/
RTC Regular time cost /20/
OTC Overtime cost /30/
hrs Working hours per period /192/
OTL limit of overtime hours per worker per period /20/;

Integer variables
W(t) Workforce size for month t;

Positive variables
P(t) Production in month t
I(t) Inventory at the end of month t
O(t) Number of overtime hours worked in month t;

W0(t) initial number of workers /Jan 1250/
I0(t) initial inventory /Jan 50000/
D(t) Demand
/Jan 1000
Feb 1100
March 1000
April 1200
May 1500
June 1600
July 1600
Aug 900
Sept 1100
Oct 800
Novem 1400
Decem 1700/;

Variable TC Total cost;

Objective Objective function
Const1(t) Workforce hiring and lay off constraint
Const2(t) Capacity constraint
Const3(t) Inventory balance constraint
Const4(t) Overtime limit constraint;

Objective… TC=e=sum(t, hrsRTCW(t) + OTCO(t) + HCI(t) + MC*P(t) );

Const1(t)… W(t)=e= W0(t);

Const2(t)… P(t)=e=((hrs/LH)*W(t)+O(t))/LH;

Const3(t)… I(t-1)+P(t)+I0(t)=e=D(t)+I(t);

Const4(t)… O(t)=l=OTL*W(t);

Model HW /all/;

Solve HW using mip minimizing TC;

display TC.l,W.l, P.l, I.l, O.l;


When i run the code, all my variables such as the total cost, the workforce size and so on are zero, which shouldn’t be the case. Can anyone help me? It is due on Thursday! ={