it's sending IR signal normally, but the helicopter just doesn't lift off.
It is not the job of the transmitter to get the heli to lift off.
The transmitter only needs to transmit the commands.
Is your receiver accurately receiving the commands? You need to test this completely, without a helicopter's whirling blades causing problems.
Put serial monitor prints on your transmitter to confirm the the values your expect to transmit.
Then put the serial monitor on the receiver and confirm the commands received.
If you do not have full confidence in this behavior, you should not add it to the helicopter.
Now, PICS!!!! Helis are awesome and I need more details of your model. Not to troubleshoot the link between the Tx and Rx, but to satisfy my curiosity.
Do you have any experience flying RC helis?
Do you have any on-board stabilization? (gyros/MPU)