Hm, I had a look at your site and found a problem with the button code for "Hydro Whey" (#1206662) on your site's "products.html" page -- the name="on" and name="os" parts should actually be name="on0" and name="os0":
<form action="https://www.e-junkie.com/ecom/gb.php?c=cart&i=1206662&cl=203494&ejc=2" target="ejejc" method="POST" accept-charset="UTF-8">
<input type="hidden" name="on" value="Flavor"/>
<select name="os">
<option value="Natural Vanilla">Natural Vanilla</option>
</select>
<br/>
<input type="image" src="http://www.e-junkie.com/ej/ejadd_to_cart.gif" border="0"alt="Add to Cart" class="ec_ejc_thkbx" onClick="javascript:return EJEJC_lc(this.parentNode);"/>
</form>
However, I could not find a problem in the button code for "X3" (#1062357) and "RX-90 SlimStics" (#1080649) on that page, nor could I reproduce the problem when adding those items to a cart. By any chance are you also selling these products from another site or page where we could examine the button code?
I can also ask Development to look for anything unusual with the two transactions (one each for X3 and RX-90) I found where a flavor Variation value was not passed for the order.