Despair casting issues

Playing ES/contagion with despair as a curse. In maps with tight corridors, such as Seepage and Augry being the best examples, casting despair is not possible. Whilst I know "aiming" the curse in tight areas can be tricky, there are areas where casting simply will not occur. Rather, the character states "Out of mana" even though the cast is being made at full mana.

Unfortunately, this becomes increasingly annoying the longer one plays and leads to a lot of unnecessary deaths, particularly as difficulty scales.

Thanks in advance.