did you pre-filter it through a coffee filter?
what size is your syringe filter and have you heated the liquid prior to filtering to ease it through?

use a 25mm syringe filter, and when you heat it, it filters a little faster. At first it may seem that its not going through but this process takes a while. Be sure not to put too much pressure or else you will bust the syringe filter. If u filtered it first with a coffe filter then there should be no problems.