Dear Federico,
as discussed over email, the signal is probably cut off by the detector itself. Please take a look at the following picture:
I was curious which detector you are using - this can be seen from the measured chromatogram. As you use A/D Converter, the signal can be cut off at two sources - either at the instrument itself, or by the A/D Converter. In the A/D converter settings you can see that the range is set to 1250 mV, so if the A/D converter does the trimming, the signal will be cut off at that value. However, your signal is cut off at approximately 100 mV, so it comes already cut off into Colibrick. You can also zoom in to the cut-off portion of the signal, on sufficiently enlarged zoom on y axis you will see that the top of the cut-off peak is not completely flat, but still rises somewhat. If A/D converter cuts the signal off, you would see completely flat part of the signal
Ivan has already advised over email to check the output settings of your instrument. First option is to use different physical output connector if there is one - most older instrumentation contained Recorder output and Integrator output, which differed in the voltage range put on them (100 mV for Recorder, 1V for Integrator usually), but the profiles were usually the same so this might not help you in the end (you would get ten times higher signal, but cut off at 1V instead of 100 mV).
The second option is to play with the output settings of the detector itself. For that you will have to go into the configuration of your HPLC detector an look for some settings that influence analog signal outputs - usually it is something like Output Range or similar. Some devices have double settings for this, where you set up output range (you want to have it as high as necessary) and signal attenuation (which on the other hand may need to be reduced).
Please let us know what detector you use, and if you succeeded in finding the settings.
Daniel