VRP model and code

m.modelSense = gp.GRB.MINIMIZE

m.setObjective(gp.quicksum(c[i, j] * y[i, j] for i, j in A1))

m.addConstrs((gp.quicksum(y[i, j] for j in V if (i, j) in A1) == gp.quicksum(y[j, i] 
for j in V if (i, j) in A1) for i in V), 
name='E7')

m.addConstrs((gp.quicksum(y[i, j] for j in V if (i, j) in A1) == 1 for i in V), 
name='E9')

m.addConstrs((gp.quicksum(f[j, i] for j in V if (j,i) in A1) - gp.quicksum(f[i, j] 
for j in V if (i, j) in A1) == d[i]for i in S), 
name='E10')

m.addConstrs( (f[i,j] <= Q1* y[i, j] 
for i in V for j in V if (i, j) in A1),
name='E11')

The issue is that a solution exists only when Q1(vehicle capacity) can satisfy the total demand of all points. To simplify the model, I did not set a limit on the number of vehicles k. However, why doesn't the model distribute the delivery across multiple vehicles?

I would greatly appreciate your help.

Thank you very much!