Carpet not tucking under door bar

My suggestion would be to buy I deeper door bar unless it’s due to the underlay causing the carpet to be higher then the bar causing it to not easily tuck under. To solve this you would need to remove the door bar put something underneath to raise the bar giving better access for carpet. The only other reason I can think of is the carpet has been cut short.

If the wrong bar has been used this is why its not holding

They could be using the incorrect bar as, any carpet regardless of underlay should fit into the suitable door bar. Hence why they’re created

There is absolutely no reason, it shouldn’t fit unless carpet has been cut short and incorrectly

Thank you

Change your fitter

If carpet is mega thick appropriate thresholds should be used

It depends on what door bar has been used but as a normal rule of thumb there is always something to do the job. A picture of said area would be beneficial