As Airburst noted, "sealed bearings" can be a misnomer as almost all headsets have rubber o-rings or similar "seals" even if they use loose balls or balls in retainer cages. Your description "gets stuck in the center position" implies loose or retainer held balls that have "brinneled", that is the balls have worn matching divots in the races and developed "index steering". Typically, true cartridge bearings aren't suseptable to this problem.

If your headset uses retainer held balls you may be able to extend its life somewhat by discarding the retainers and adding a couple of more balls to each race. That keeps the balls from aligning with their divots and will smooth out the headset for a while. Otherwise, a new headset is called for.