how have you tried it? you need to be on flat ground, or going downhill and you need guard mode on, patience is a key, dont break testudo until youve split the phalenx or at least gotten the center units to switch to sword.
secondly what units are you useing VS the testudo

Sacred Band
stat_pri 12, 8, no, 0, 0, melee, simple, piercing, spear, 25 ,1
stat_sec 10, 4, no, 0, 0, melee, simple, piercing, sword, 25 ,1
stat_pri_armour 11, 7, 5, metal
vs
roman legionary cohort ii
stat_pri 13, 4, pilum, 35, 2, thrown, blade, piercing, spear, 25 ,1
stat_sec 9, 4, no, 0, 0, melee, simple, piercing, sword, 25 ,1
stat_pri_armour 12, 5, 5, metal
yes the romans will get owned, the units arent equal even not in phalenx sacred band will own

try vs
Urban Cohort
stat_pri 18, 4, pilum, 35, 2, thrown, blade, piercing, spear, 25 ,1
stat_sec 14, 4, no, 0, 0, melee, simple, piercing, sword, 25 ,1
stat_pri_armour 12, 7, 5, metal

look for a phalenx unit that has a lower armor and secondary attack then legionaires cause thats what matters once the testudo does its job, and i never claimed that it would allow them to beat better units, spartans will kick legion butt out of or in phalenx