brainjs predict the next value

I'm working on a neuronnal network and I'm trying to do prediction. For that I have an array of array that containt value and I would like to know what will be the next one.

Just to practice I did something really simple but it don't work (the value returned is wrong), Can you explain me wait I'm missing ?

const NN = new brain.recurrent.LSTMTimeStep({
    inputSize: 2,
    hiddenLayers: [10],
    outputSize: 2,
});

let data = [
    [1, 2],
    [2, 4],
    [3, 6],
    [4, 8],
    [5, 10],
    [6, 12],
    [7, 14],
    [8, 16],
    [9, 18],
    [10, 20],
    [11, 22],
    [12, 24],
    [13, 26],
    [14, 28]
];

const config = {
    log: true,
    logPeriod: 100,
    errorThresh: 0.01,
    iterations: 4000
}

NN.train(data, config);
let output = NN.forecast(data, 1);
console.log(output)

On this I want the result to be [15, 30] but it keep return lower value.

Thanks a lot

Changing the number of hidden layer and add some iteration seems to be the solution, my AI wasn't wrong, just not accurate enought

const NN = new brain.recurrent.LSTMTimeStep({
    inputSize: 2,
    hiddenLayers: [2, 2],
    outputSize: 2,
});

let data = [
    [1, 2],
    [2, 4],
    [3, 6],
    [4, 8],
    [5, 10],
    [6, 12],
    [7, 14],
    [8, 16],
    [9, 18],
    [10, 20],
    [11, 22],
    [12, 24],
    [13, 26],
    [14, 28]
];

const config = {
    log: true,
    logPeriod: 100,
    errorThresh: 0.01,
    learningRate: 0.001,
    iterations: 40000000
}

NN.train(data, config);
let output = NN.forecast(data, 1);
console.log(output)

With lower learning rate and more iteration the NN have more time to fit perfectly.