There can be a number of reasons, some companies just don't fall into scope. For example, LEO cannot grant aid anything in the gambling industry.
Secondly, they have a defined budget to live within for grants and in particular towards the end of a year, if that budget is used up, they may defer or tell you to apply next year. That budget also drives priority and under their rules, priority can be given to firms with export potential. One of the aims of a LEO is to help a business grow so it can be handed over to Enterprise Ireland
Thirdly, there is what is called displacement. LEO will quite often not grant aid a new start up if it means another firm 5 miles down the road will end up going out of business or laying people off as a result. That's the main reason why LEO very rarely grant aid Professional Services set ups.
You also need to ensure your tax affairs are in order and satisfy LEO that you have the skills and capacity to handle the business. I've sat on the approval committees in the past, the most common reasons for refusing a grant was because the entrepreneur would not listen, thought he knew better and in my experience, 90% of those companies and idea went to the wall within a couple of years.