Your formula and method are incorrect. 37 grams of Paraformaldehyde + 100 ml of water will not make a 37% solution. It will be more dilute than that. See here:

Take about 50 ml of warm water and add a drop of 37% Sulfuric Acid to it. This is common battery acid. Then add 37 grams of Paraformaldehyde to that in small amounts while stirring. It should dissolve. If it does not, add another drop of acid. When all solid is added, dilute this to 100 ml and this will make a 37% (weight / volume). Be careful as Formaldehyde is a gas and can overcome you in a closed space. Wear eye and hand protection and use a breath mask or work outdoors if possible.