some enlargers using dichroic filters won't go all the way to a real grade 5. Others will. If using dichroic filters I'd suggest trying and Ilford G5 filter. It may well give higher contrast.
Also use new paper and definitely not old stock.
Also get some benzotriazole and mix up a 1% solution. Add 5ml of 1% solution to one litre of working strength developer. This will give contrast a bit of a kick. Add more if necessary.
Also warming up developer will give a little boost to print contrast.
Also under exposing and over developing can sometimes give a boost in contrast, especially on graded papers.
do all the above and you should get a decent rise in contrast.