Medela freestyle has 2 phase suction, when you first turn on you will be at phase 1 which mimic baby's fast sucking and hence may not be that strong. But once you press the "let-down' button, you should be able to feel stronger suction.
There could be several other reasons why your pump's suction is not strong and one reason is it is not completely vacuum or suction to your breasts.
If you have the instruction manual do go through it.
Breast pump motor usually get weaker due to wear and tear hence if your pump is more that 1 year and has been highly utilised before hand, chances r you might need to service it to maintain it.
If its out of warranty, servicing the pump is expensive which i just experienced it myself. The medela service centre charges $140 for servicing of freestyle pump excluding any change of parts.