The decision to form the HRE becomes visible for Catholic kings in the traditional HRE areas once either the Karlings have formed the Carolingian Emprie first and then had it fall, or the Karlings have lost control of both France/Neustria/West Francia and Lotharingia/Austrasia/Middle Francia (as in, both of these titles cannot have Karling holders).
This is probably your problem - if you broke away from the Karlings but did not overthrow them, they probably don't have the necessary land to form the Carolingian Empire, since that requires them or a vassal to hold the King titles of Saxony and Bavaria, or (if those kingdoms don't have kings) for them or a vassal to completely control the duchies of Saxony and Bavaria. So you can either re-vassalize yourself or somehow get the Karlings off those thrones.
Once the decision is visible you need to fill its requirements as well (be at peace,1000 prestige, 100 piety,150 realm size, hold the kingdom of Austrasia/Lotharingia or Germany/East Francia, control 7 duchies in Germany, don't be excommunicated)
