Bro maybe you ur marker reach "due date" or you have used your marker without shaking them even last time ~ Most likely the metallic flakes gone d. But, my bro last time did bought a white marker ~ only can use a little bit then only left the transparent liquid flow out ~ wasted $$

for acrylic, u can use this wash detergent called Kuat Harimau (made in alor gajah, melaka) 

based on my experience, even a fully cured paint job, will easily removed... just soak the model into a warm bath of kuat harimau...depending on the paint cured time, sometimes in just 1 hr u will see the result..then slowly brush the surface with an old toothbrush... sometimes tough paint job require to soak in overnight.. hope this help..this is the cheapest method i developed...hehe