Maybe you were 'locked-in' with a different character without realizing it. Here are the dialogue options from the wiki
For a male Shepard, a love interest is
"locked in" once you express interest
at the following dialogue points:
Jack: "I want to talk about us."
To prevent locking-in, say "Never mind".
Tali: "I want to talk about you." at
the point where she beckons you toward
the drive core. To prevent locking-in,
say "So you're not interested?"
followed by "I need to think about
Miranda: "Can we talk?" at the
point where she says "I don't know
what this is, if it's just stress, or
blowing off steam, or..." you can not
opt out; every option results in
either becoming locked-in, or the
relationship being broken off
permanently. Hope you saved before the
At this point you know
that Shepard has successfully 'locked
in' that romance.
Shepard can still
express an interest in other crew
members. However, the new love
interest will 'challenge' Shepard's
love-triangle and force Shepard to
dump the 'locked-in' option in order
to be with them. Once a potential
relation is at the 'challenge' part,
there is no way to express disinterest
in them no matter what Shepard choose.
However, if Shepard dumps someone who
is 'locked in', that romance option
will never be available. A romance
scene with the 'locked-in' character
will be shown when the Normandy SR-2
proceeds through the Omega 4 Relay.
Although it is possible to have a
sexual encounter with Jack before
this, it does not count as a romance.
If Shepard and the love interest
survive the suicide mission, it is
possible to invite him or her up to
the Captain's quarters to either sit
with them on the couch, or lie with
them on the bed.