Welll a full-scan is neither good nor bad, it is just a thing. You can promote index usage in a number of ways, and for a single query the simplest may be to just use hints to promote index usage.
However, take the trouble to measure and document performance with and without the indexes being used, and keep an open mind on whether you ought to be using them or not. If you can conclude that there are clear benefits to using the indexes then you can look to see why the optimizer is not doing so. It may have bad information about the tables, the indexes, the data, the system ... but the first step is to see whether it is right or wrong in the first place.