Why have birds almost totally stopped coming to our feeders? They were prolific 4-5 yrs ago but have steadily declined. Seed/nuts clean, changed regularly. Feeders hanging under crab apple tree. Now only Great and Long Tailed tits, occasional Bluetit.

It is difficult to say and I offer the following comments in good faith. No amount of food offering will help if there is not the habitat in the surrounding areas to support birds. If you are urban take a slow walk around your neighbourhood. Have front gardens lost hedges and plants and been tarmac'd over? A small privet hedge can provide a potential roost site over winter and a nesting site in summer. What has happened to the back gardens? have they had major makeovers with shrubs and fruit trees removed to make way for trampolines and decking or the newest horror artificial grass (that stuff should be banned).
If you are rural have there been changes in farming practice? or perhaps green and brown field sites lost to development.
Mature oak trees are great for providing all sorts of insect food but they are not so great for nesting in.
If birds cannot safely roost in dry protected areas over the winter no amount of supplementary feeding will see them through to spring to breed.
It's really complex and many factors will feed into the problem.