Hi ginky -- my thread is right next to yours at the moment -- called idiopathic thrombocytopenia (basically the medical term for low blood platelets with no known cause). My dog had low blood platelets but also showed other symptoms like lethargy, low appetite and lameness.
In the end she had the tick disease anaplasmosis. Like hazelrunpack has said many other tick borne diseases can also cause low blood platelets. Other reasons platelets can drop are immune disease, ITP (which my dog was misdiagnosed with), vaccines or other medications, chemicals or toxins.
That being said, platelets naturally vary a little bit within the normal range. For some labs, >180,000 is normal, for others >200,000. Also, if it takes too long for the sample to arrive in the lab, platelets clump together, and the machine artificially reads less platelets in the sample.
Do you know the actually number of the platelet count? If your dog is not experiencing any other symptoms you might want to test again to confirm that the platelets are still low. Perhaps do a tick panel like hazelrunpack has suggested. With tick disease, if a dog does not show clinical symptoms, some vets will not prescribe antibiotic treatment. Many dogs test positive for tick disease and do not show symptoms if they are able to fight off the infection themselves, while some dogs get very sick. Some tick disease can remain hidden in a subclinical phase and reappear down the road when the dog becomes older or ill. So, it is good to rule out with testing.