It could be that you are taking a thick hair section to straighten. This will affect the quality of straightening and you may need to repeat the process a few times.
Try taking a small section of hair, not more than 5 to 6 cm wide, to straighten your hair in one attempt.
For Philips SenseIQ straightener, you may also try "Fast" mode or choose a different manual setting to get faster straightening results.