onchange and onclick not work consistantly

I'm trying to scrape data from a webpage. In this process, I need to change one of the dropdown of that web page. I was able to do that. But when I manually change the dropdown, webpage changes it's content like images and some other text. But when I do that using VBA it doesn't do that. I tested my code several times. So if I run the program 10 times it will change the content 1 or 2 times. So I'm confused about what happening

This is the HTML of that dropdown area.

<tbody>
<tr>
<td class="label"><label for="pa_product">Product</label></td>
<td class="value"><select id="pa_product" name="attribute_pa_product" data-attribute_name="attribute_pa_product">
<option value="">Choose an option…</option>
<option value="air-xs-2-octo-16-inch" class="attached enabled">AIR XS 2 Octo 16 Inch</option>
<option value="air-xs-2-octo-18-inch" class="attached enabled">AIR XS 2 Octo 18 Inch</option>
<option value="air-xs-2-octo-20-inch" class="attached enabled">AIR XS 2 Octo 20 Inch</option>
</select> <a class="reset_variations" href="#reset">Clear selection</a></td>
</tr>
</tbody>

And this is the code I have developed. I think onchange and onclick work rarely.

    Do
    DoEvents
    Loop Until objIE.readystate = 4

    'Body(HTML)
    'MsgBox objIE.document.getElementById("tab-description").outerHTML, , "OHTML"
    'MsgBox objIE.document.getElementById("tab-description").innerHTML, , "IHTML"
    'MsgBox objIE.document.getElementById("tab-description").innerText, , "ITex"
    WS.Range("D" & i).Value = objIE.document.getElementById("tab-description").outerHTML

    'Option 1 value
    'MsgBox objIE.document.getElementById("pa_product").outerHTML
    'MsgBox objIE.document.getElementById("pa_product").innerHTML
    'MsgBox objIE.document.getElementById("pa_product").innerText


    objIE.document.getElementById("pa_product").selectedindex = 1
    Application.Wait (Now + TimeValue("0:00:01"))
    'modify the web page for that selected item
    'For x = 1 To 5
    'objIE.document.getElementById("pa_product").selectedindex = 1
    Application.Wait (Now + TimeValue("0:00:03"))
    objIE.document.getElementById("pa_product").FireEvent ("onchange")
    objIE.document.getElementById("pa_product").FireEvent ("Onclick")
    objIE.document.getElementById("pa_product").FireEvent ("onmousedown")
    'Next x

    Application.Wait (Now + TimeValue("0:00:03"))

    WS.Range("J" & i).Value = objIE.document.getElementById("pa_product").Item(1).innerText

Thank you very much.