php GD library sometimes generate blank image - php

Question

I trying to generate a signature from a name using the GD library, and return the png image in base64 format.
With the following code, the function correctly returns the base64 image with the signature:
<?php
function genSignature($name){
// The text to draw
$tamano = 15 * strlen($name);
$fuentes = [
'../fuentesFirma/HomemadeApple-Regular.ttf',
'../fuentesFirma/Allura-Regular.ttf',
'../fuentesFirma/DawningofaNewDay-Regular.ttf',
'../fuentesFirma/HerrVonMuellerhoff-Regular.ttf',
'../fuentesFirma/LaBelleAurore-Regular.ttf',
];
$fuente = $fuentes[array_rand($fuentes, 1)];
// Create the image
$im = imagecreatetruecolor($tamano, 60);
// Create some colors
$white = imagecolorallocate($im, 255, 255, 255);
$grey = imagecolorallocate($im, 128, 128, 128);
$black = imagecolorallocate($im, 0, 0, 0);
imagefilledrectangle($im, 0, 0, $tamano, 60, $white);
// Add the text
imagettftext($im, 20, 0, 0, 30, $black, $fuente, $name);
ob_start();
imagepng($im);
$buffer = ob_get_clean();
ob_end_clean();
return base64_encode($buffer);
}
echo genSignature('María'); //Commented line when calling from another script, for testing only.
But if I include the file with the function, and call it from another script, it always returns the same image, a white square (base64). I don't understant where is the problem..
Other script:
<?php
include_once('generarFirma.php'); //in same folder
//....
//some operations...
$signature = genSignature('María');