aum manthra 

1 manthra with meaning tends to dwell at the thinking level, though at subtlest, reaching all lokas /all devas, even, 

but will not help easily in going beyond to parabrahma, so has less power .

but by chance it may stumble upon parabrahma level, so benefits will be less /limited/--------------used for grosser practices like, japa recitation

2 manthra without meaning enables one to go beyond easily quickly, and absolutely

so effects are more profound, in shortest time----------------------used for meditation 

cosmic entity may note why results are not seen, that is why initiation from qualified teacher is a must

in any case the journey has to be real, walk on track, lest a jungle sure