I started my career as a chemist with Loctite Corp., and we used to manufacture our own monomers. Quite frankly, it was a problem until we discovered how to stabilize them in a consistent fashion. This was achieved by using the correct levels of free radical stabilizers and metal chelators. These adhesives contain initiators and accelerators, which are usually hydroperoxides and aromatic amines. When the adhesives are applied and come in contact with metallic surfaces like iron or copper, these initiation systems generate free radicals that polymerize the monomers to give cured products.
In your case, it looks like your monomers may have different stabilizers or stabilizers at different levels. The most common free radical stabilizers are hydroquinone or methoxy hydroquinone (MEHQ). Check the certificates of analysis from your suppliers; they should list the stabilizer content.