Party members in the back row cannot attack enemies, unless:
- They're wielding a melee weapon that's labelled as a "Reach Weapon", e.g one of the spears those skellingtons drop.
- They're a Rogue with 12 points in the Assassination skill, which lets them attack from the back row.
- They're not wielding melee weapons, but are using ranged weapons (like rocks, bows & arrows, throwing knives, or spells)
So, in order to make sure all your party members can attack: put spear-shakers or rock-hurlers in the back row, and bare fists or melee weapons in the front.
If you had a character in the back row with a Reach Weapon who still couldn't reach an enemy, then something weird is going on; maybe the enemy was too far away in general? There might be some more mechanics involving Reach Weapons (like maybe a character in the front with a spear can attack at a greater distance than normal) that could be giving you the wrong impression about effective weapon ranges.