Not being able to eat properly with new dentures is very common.
A lot of it has to do with the overall change of size of the new dentures and the teeth used in making them.
It’s similar to receiving a new pair of shoes and going for a walk or dancing.  They may fit well and be right for you, but you and your mouth have to get used to them This only comes with time, and it varies from patient to patient.
After awhile if you still can’t eat properly, the dentures have to be examined, it could be that they are not right for you, they may be either to high or to low, the angle of the chewing surfaces (cusps) of the teeth may be to shallow or to high.
We as humans need sharp angled cusped teeth to be able to chew properly, not flat ones where chewing becomes a chore and uncomfortable, especially as we get older.
This is a very good question, and you should give your new teeth a proper amount of time to get used to, but if it doesn’t improve then the dentures need to be examined and altered to correct the source of the problem.
This could mean to rebase or redo them to make them fit better , and/or reshape or change the actual teeth , allowing you to eat comfortably and in good time.
Thank you,