Unable to fill certain fields inside the DOM of a website

Question

Hello,
I am trying to create a form filler program and came across a gap:
some of the sites's DOM fields simply can't be edited - for example:
https://login.bankhapoalim.co.il/cgi-bin/poalwwwc?&bank=414
the fields - "userid" and "id" can't be edited using the following code:
 
if (i_Webbrowser.LocationURL == "https://login.bankhapoalim.co.il/cgi-bin/poalwwwc?&bank=414")
{
     HTMLInputElementClass input = GetInputElement("userid", i_Webbrowser);
     input.value = "someValue";
}

does someone have any explanation/suggestions?
it is not the httpS issue since gmail fields can be filled.

thanks!!!