i think the thing with the c4 really depends on where you plant it
when i was doing the c4 badge i always used 3 c4 on different locations and still got kills for each.. maybe my guys had already been injured and low on health or it was just bad luck for you.
but still do try some different locations they are easy to spot on solid ground and prone to get blown up by grenades on chokepoints so instead try planting them on the left and right of doorframes at about shoulder height ,hallways and behind corners and on capture and hold objectives work good aswell