We have added code to the _StoreHeaderNavigation so that the count of items in the basket displays. The _StoreHeaderNavigation is part of a new widget board for the header instead of using AC header because client wanted a custom megamenu and other things in the header.
Part of the code was already there-
var basketItemsCount = BasketHelper.GetBasketItemsCount();

We just added code later on to display the count
<li><a href="@Url.Content("~/Checkout/Basket")">
<i class="fa fa-shopping-cart"></i>
@GetResourceString("retail.storemenu.cart")
@if (basketItemsCount > 0)
{ <text>(@(basketItemsCount))</text>}

</a></li>

Sometimes this updates, sometimes it doesn't. If I add to the cart from the category page, it updates. If I add from the product page, it doesn't update and if I do refresh page in the browser using right mouse button, it doesn't update.
On the product page, there is a tab that shows upsell products and if I click add to cart there (on the button that works, see other post), the header count updates.
Why would this be and is there a solution?
Thanks

The header is quite customized. Let me take portions of it off at a time and troubleshoot that way first.
Thanks for your help.

It turns out that part of the problem was that someone had put caching on a header menu.