Its only because you just made it, Tanks aren't really tanks at the begginning their more like scrappers until you get to your teens, i say stick it out till like lvl 10 and see if its any better 
my inv/axe was the same
Invul is still ok as secondaries dont really affect it, but mace wont get much more powerful later on either.
Stone/Mace is far more gimped 
okie dokie i think ill ride it out till 20 if it havent changed by the time i get stamina ill put it down as a failed char 
Hmm,, all the tanker sets start slow.. and if you're expecting something to suddenly change at 20 you are going to be disapointed.
I have a inv/axe running at 37 now, axe is somewhat comparable (bitty tiny less) to mace. Its just hanging out till 22 SO's, i more made it a scranker so by taking more passive's you should do even better.
Its a great happy damage dealer, but versus high-SL resisted mobs.. thats plain annoying.
50)Sinergy X/(50)Mika.
(50)MaceX/(50)Encore
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moog i was told that at lvl 20 things start looking up cause youve just got stamina and you can keep the attacks rollin? with a few end red enhancments ofc
It's L22 and SO's that makes the real difference, before that your survivability is much lower.
k ill try and get to 22 but im not promising anything i want to, but i always get dragged back to my mm he is so coooool
i felt the same about my inv/em tank. that all changed at lvl24..........respec'd with tough and weave. slot ur powers correctly and you wont give a damn about how much damage your putting out because all you will be doing is laughing about the stupid ammount of aggro you will be taking. i dont care what anyone says about inv....leave out res nrg and res ele you will still be one of the best alpha strike takers in the game without them.
last night i made a mace/invun and compared to all the other tankers ive tried i seemed to do hardly any dmage and my helth went down at an unusual rate for invun