При определенных значениях не выполняется цикл

Dim k1 As Double, k2 As Double, k3 As Double, k4 As Double, oshibka As Double, max As Double, f_max As Double, n_max As Double, x As Double, proizv_x As Double, proizv_a As Double, proizv_b As Double, a As Double, b As Double, f As Double, proizv_max As Double
 
Dim fun As String
 
Private Sub Command1_Click()
 
k1 = CDbl(Text1)
k2 = CDbl(Text2)
k3 = CDbl(Text3)
k4 = CDbl(Text4)
oshibka = CDbl(Text5)
a = CDbl(Text6)
b = CDbl(Text7)
 
proizv_a = k2 + 2 * k3 * a + 3 * k4 * a * a
proizv_b = k2 + 2 * k3 * b + 3 * k4 * b * b
x = a - (proizv_a) / ((proizv_a) - (proizv_b)) * (a - b)
proizv_x = k2 + 2 * k3 * x + 3 * k4 * x * x
iter = 1
 
Text9 = "¹ / Òî÷êà / Çíà÷åíèå ôóíêöèè / a / b / Ïðîèçâîäíàÿ:" + vbCrLf
While Abs(proizv_x) > oshibka
If proizv_x > 0 Then
b = x
proizv_b = k2 + 2 * k3 * x + 3 * k4 * x * x
iter = iter + 1
End If
If proizv_x <= 0 Then
a = x
proizv_a = k2 + 2 * k3 * x + 3 * k4 * x * x
iter = iter + 1
End If
x = a - ((proizv_a) / ((proizv_a) - (proizv_b)) * (a - b))
proizv_x = k2 + 2 * k3 * x + 3 * k4 * x * x
i = i + 1
f = k1 + k2 * x + k3 * x * x + k4 * x * x * x
Text9 = Text9 + CStr(i) + " / " + CStr(Round(x, 3)) + " / " + CStr(Round(f, 5)) + " / " + CStr(Round(a, 3)) + " / " + CStr(Round(b, 3)) + " / " + CStr(Round(proizv_x, 8)) + vbCrLf
Wend
 
n_max = iter
max = x
f_max = k1 + k2 * x + k3 * x * x + k4 * x * x * x
proizv_max = k2 + 2 * k3 * x + 3 * k4 * x * x
fun = "(" + CStr(k1) + ") + (" + CStr(k2) + ")x + (" + CStr(k3) + ")x^2 + (" + CStr(k4) + ")x^3"
 
Text8 = fun + vbCrLf + "Êîëè÷åñòâî èòåðàöèé: " + CStr(n_max) + vbCrLf + "Òî÷êà ìàêñèìóìà: " + CStr(max) + vbCrLf + "Çíà÷åíèå ôóíêöèè â ìàêñèìóìå: " + CStr(Round(f_max, 3)) + vbCrLf + "Çíà÷åíèå ïðîèçâîäíîé â ìàêñèìóìå: " + CStr(Round(proizv_max, 8))
 
End Sub