Confusion over MPS LLC design tool

Using HR1001A.
I need to iterate a transformer design for LLC converter, to change it from 48V 8A to a higher voltage, lower current.
The transformer is an integrated design is made by a specialist company and is a standard part on general sale, so it must work… right?
I put the design figures into MPS LLC design tool which basically spat it out and said it can’t work.
Any clues?
The design is typical. 400V dc input. centre tapped secondary.
Primary is 32 turns
Secondary is 8+8 turns
Primary inductance (secondary open circuit) is 690uH
Primary inductance (secondary shorted) is 115uH
Resonant cap is 56nF
Operating frequency is stated as 46kHz to 88kHz
Output 48V 400 watts

I have also run this is MPsmart simplis and that seem to work in a reasonable manner.
The query is over the tool on this MPS website.

Hi Jerry,

Can you post a snapshot of the values you had entered into the LLC tool? I would like to know the input values.

Hi, thanks for the reply.

I should mention the transformer is integrated type, so whilst I have used Ln of 6 (690/115), there is an argument that it should be 5 ((690-115)/115), but even 5 gives a problem.
In both cases, insufficient gain.