Hi, Pablo it sounds like your are not getting any satlites singles to your
nav/computer, check your aerial connection at the back of the nav/computer, or it could
the map disc as not been loaded correctly. your NAV system requires the Almanac to be loaded into NVR. This is basically a course orbital parameter lookup table that says what 'birds' are available and in what position in the sky, for a given date/time. Almanac data is only used for the initial startup to give the system an indication of what birds are in sight and once one is locked, the system will then download the ephemeris data for that particular bird which is a much more accurate and up-to-date table of orbital data.
The Almanac is only valid for a certain length of time and if the position of the GPS has moved significantly since the almanac was loaded, it will have to do a 'sky search' to pick up a good bird to enable an almanac download. Once it has this and can collect sufficient birds with valid ephemeris data, it will be good to provide positioning info.
Now, some GPS systems that require a sky search can take 30 minutes or more to acquire a good bird that is up long enough to download the almanac, so you really need to go to a nice open field with a good view of the sky and horizon (if possible), leave the system powered up for 30 minutes to an hour and in that time, if there is nothing wrong with the system, you will get a good almanac downloaded, from then on things should sort themselves out.