It could well be that your machine needs cleaning. At least once a year you need to give it a thorough clean, removing all the needles and cleaning all the fluff that has accumulated inside the needle bed. A vacuum cleaner is good for this and an old needle to hook out any stubborn dirt. Also clean under the carriage, brushing out all the old fibres and oil. Use surgical spirit to clean off the blackened oil before applying new oil on a cloth. Check all the needles to make sure none are damaged.
Check the sponge bar in the main machine, it might need replacing.
Make sure the carriages are seated properly on each bed and that the sinker plate or ribber connecting arm is flat against the carriage before you tighten the thumb screws.
This should give you a starting point, hope it helps,