If the soil is quite wet it may be Phytopthora sp., some sort of bleeding canker or root rot. Is there any ooze from near the base of the trunk or any black mould under the bark?

Or Vine Weevil grubs have eaten the roots over winter - I lost an Acer to these nasty little maggots once; more likely in pots or light soil types.

Both may prove terminal.

Two suggestions, not an exhaustive list.