can't generate a Random value?

I´ve made a code on visual basic for a wind calc. but can't run a random value, as the results are not random. I´ve run it twice just to check if values are random but not at all. Lot of time working on this code and need some help pls

Please see my code below (if you spot others mistakes, please let me know, much appreciate)

For j = 1 To n_p
    ti = 0
    For i = 0 To n_t
        ti = delta_t * i
        Sum2 = 0
    
        For m = 1 To j
            Sum1 = 0
            For l = 1 To n_f
                w_lm = (l - 1 + m / n_p) * delta_w
                C = Exp(-lbd * w_lm * delta_L / (2 * Pi * U_m))
                
                If j < m Then
                    G = 0
                ElseIf m = 1 Then
                    G = C ^ Abs(j - m)
                ElseIf m <= j And m >= 2 Then
                    G = C ^ Abs(j - m) * Sqr(1 - C ^ 2)
                End If
                
                fu = L_u * w_lm / (2 * Pi * U_m)
                S = 4 * fu * S_v ^ 2 / (w_lm * (1 + 70.8 * fu ^ 2) ^ (5 / 6))
                                
                fi = Rnd() * 2 * Pi
                
                Sum1 = Sum1 + Sqr(S) * G * Cos(w_lm * ti + fi)
            Next l
            
            Sum2 = Sum2 + Sum1
        Next m
        
        Vi = Sqr(2 * delta_w) * Sum2