Payment plugin not working newly registered users.

Hi, been having this very wierd issues, where customers are not able to pay for their orders (SAGEpay plugin) instead of being directed to sagepay's server, user is immediately seeing "thankyou for your purchase" message. In the back-end i can see these orders 'created' but not confirmed or shipped.
I have gone through all the settings i can find, including restrictions set in the sagepay plugin, shipping methods etc, but it still happens.
Now I seem to have identified that this problem specifically occurs with newly registered users. If the user logs out and back in again they are then able to pay.
Is this a bug or a configuration weirdy? Any ideas

ps my client has bought hikashop essential, even though we haven't installed it yet.

Sure,
the shop is at www.uvpaint.eu

I can pm you login for the back end if you need it

Thanks for your help

cache plugin is disabled, i don't know if there is any other cache activated... is this a setting in hikashop ?

joomla cache is also disabled in the global config....

ah! could be because i don't have the payment method selection appearing in the basket? havent included it because i have "autoselect payment method" switched on, and user should never have to make a choice about this....

Thankyou, that certainly does seem top fix it!

I am still a little puzzled as to why this step can't be omitted when only one payment option is available, perhaps and auto submit would be useful for this?

It doesn't seem to be adversely affecting the users though so this fix is plenty for now, thanks again