I'm fairly new to GMing (I've done four sessions now of BC and previously have run some D&D) and am having difficulty with an issue I haven't really experienced before. In any game I've GMed before I have never had the issue of my players not being able to take down the enemy and indeed often had to upscale my encounters to try and deal with them.
However, my new group of heretics is quite the opposite. The group is three chaos marines and one human(two forsaken, a champion, and a psyker). The group has proven to somehow manage to be entirely incapable. My players built their characters fully and only around the RP aspect of their character, disregarding stats/boosts/etc. in order to craft a better character. In this I'm extremely happy because I have dedicated RPers. However, they totally suck in any type of combat situation.
I had assumed that three marines and a psyker would prove to be a powerful team but thus far they've proven entirely underpowered. I had hoped maybe they had been unlucky their first few encounters but that hope is fading. The psyker seems to have an unnatural trait for rolling perils every time she casts, the champion has already burned both his infamy from being mauled, and I'm almost positive that were I to ask the forsaken to shoot a barn they would manage to ricochet the bullets off somewhere and kill themselves.
My question is, how do I handle severely underpowered (or at least amazingly incapable) PCs? They had trouble dealing with a single enemy marine, and when they ran into the rest of the squad it was pure hell. I shudder to think what will happen in this upcoming session where they have set themselves up to raid an enemy warlord's stronghold. Thanks in advance