This is difficult to answer! We agree this is a very common symptom. It is not due to air/wind, which most patients think.
Please have a look at these useful RG link.
Article Abdominal Bloating: Pathophysiology and Treatment
Article Pathophysiology, evaluation, and treatment of bloating: Hope...
Abdominal bloating in IBS occur due to frequent muscle contraction that lined the wall of the large intestine
Genetic, environmental, and psychological factors seem to be important in the development of IBS. Studies have shown that IBS has a genetic component even though there is a predominant influence of environmental factors.
There is evidence that abnormalities occur in the gut flora of individuals who have IBS, such as reduced diversity, a decrease in bacteria belonging to the phylum Bacteroidetes, and an increase in those belonging to the phylum Firmicutes. The changes in gut flora are most profound in individuals who have diarrhoea predominant IBS. Antibodies against common components (namely flagellin) of the commensal gut flora are a common occurrence in IBS affected individuals.
Chronic low-grade inflammation commonly occurs in IBS affected individuals with abnormalities found including increased enterochromaffin cells, intraepithelial lymphocytes, and mast cells resulting in chronic immune-mediated inflammation of the gut mucosa. IBS has been reported in greater quantities in multigenerational families with IBS than in the regular population. It is believed that psychological stress can induce increased inflammation and thereby cause IBS to develop in predisposed individuals.
Irritable bowel syndrome (IBS) is a group of symptoms—including abdominal pain and changes in the pattern of bowel movements without any evidence of underlying damage.
Exact mechanism unknown but possibly factors like visceral hypersensitivity, altered gas handling in gut (including motility, secretion), excess fibres, altered gut flora interplay to different extent and in various permutations and combinations to produce the symptom. CNS may also be involved as in other functional disorders.
Falah Shari - Clinically we use ROMA IV criteria to assess irritable bowel disease and severity. I enclose this simple website which will give you a good idea.
https://irritablebowelsyndrome.net/clinical/new-rome-iv-diagnostic-criteria/