Hi,
Please check you do not have restrictions in your plugin configuration (No "zone", no "currency" selected, no "shipping" selected).
These fields are restrictions, so it could hide the payment method if the restriction is not validated (if the customer does not have the right currency, etc).
Otherwise, please contact the iDeal team.
The plugin as not be implemented by our team so we can't provide useful support on it.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.