Check the pH.

I guess your hematoxylin deeping time is too much and your eosin is not balanced. Check back your protocol.

Honestly added - also proposing to do troubleshooting by facts (which needs at least more detailed information about the specimen(s), its / the processing -starting from fix to embedding and sectioning - and for sure also the SOP of the SIGMA -staining -?kit?- ) :

when logged in to ResearchGate browser search for | TRAP Stain | [or also

| Protocol Tartrate resistant acid phosphatase | ] and other keywords/phrases

with the full text in menu-rectangle |SEARCH for ......| (see top middle of the/this particular page )

and find results for / in Section

"Publications" cf: https://www.researchgate.net/search.Search.html?type=publication&query=TRAP%20stain

"Questions": cf.: https://www.researchgate.net/search.Search.html?type=question&query=TRAP%20stain

and last, but not least: in

"Projects": cf.: (ONLY ONE result with similar - if not the same! - problem):

https://www.researchgate.net/search.Search.html?type=project&query=TRAP%20stain

Use of Hematoxylin as counterstain seems to be also a bit tricky, cf.: https://www.researchgate.net/post/Hematoxylin_for_TRAP_stain

From such sources you should get most rapidly some 'impression' about what was going wrong with your staining...

Information about "how to" / a staining protocol (s) as provided by suppliers not necessarily describe a reliable protocol for specific specimen preparation implementation (e.g. sampling, former primary fixation, washings, embedding sectioning and finally treatment of sections until mounting properly under coverglass, etc.).

Unfortunately, neither I have the SIGMA-manufacturer's staining protocol at hand nor do I have information on the steps you really did. ...so it might be easier to comment on if you include - at least- some detail of your specimen processing too.

Only for convenience I enclose here an article on the technique

'Role of tartrate-resistant acid phosphatase (TRAP) in long bone

development' by BLUMER et al, 2012 in: Mechans of Developmt 129, pp. 162-176, where the staining components as well as the displayed protocol might be at least similar to your demand

Naturally: always there might be the possibility of deteriorated dye or poor quality of components and failing of stable staining reaction(s) and products...so it might depend what the cause for your unexpected staining failure was.....

Best wishes and regards, WHM