lux ult

Sometimes when i play lux and i flash or respwan it wont let me use my ult.The last game i had with lux jax was like 100hp and my ult was in range and i flashed to come closer and ult but it didnt let me ult i spammed my R and it still didnt go off and then when i respawned it didnt let me ult again even i spammed my R.It happened to me the first time a few months ago.