Be sure you have completed it to the end with the final CLICK bringiing up the TRIAL PAY success page. Leave that Page open while you refresh a different WGT page.. sometime refresh 2 - 3 times... confirm you have your credits and close the Survey success page..
Apart from that..
In the OFFERS area - there is a link for SUPPORT - by Trial Pay
Follow their instructions, will mean pasting the email you received to them etc.
I do surveys OFTEN, unfortunately, 5% of the time they glitch up and dont pay.
1% of those times if you email Support. You will get your credits paid.
If it is a substantial amount of creds, perhaps after buying flowers etc.. Email support again and if you get no satisfaction provide all your proof as well as contact with Trial Pay support to WGT - they can 'sometimes' prod Trial Pay.