I want to know if there are any issues in PET scan? Is it a full proof method to detect cancer?

Basically there are no known issues with PET scans, other than they provide a useful means of identifying the presence and activity of some cancers. A PET scan would normally be used in conjunction with a CT scan to obtain an effective overlay of the images. As the CT will show any abnormal growth the PET scan would reveal if there are any live cancerous in the given area.
However, PET scans are not full proof as they are not capable of identifying cancerous growth that is less than about 1 cm in size. This equates to approximately 1 million (1x10^6) cancerous cells that may go undetected. Therefore, to ensure the accuracy of a PET scan, the active cancerous grouth would need to comprise of 1x10^9 cancerous cells. This would equate to a tumour size of between 1-2cm.

Hope this helps.

PET can help physicians effectively pinpoint the source of cancer. This is possible because many cancer cells are highly metabolic and therefore synthesize the radioactive glucose (sugar) that is injected in the patient prior to the exam. The areas of high glucose uptake are dramatically displayed in the scan imagery, as opposed to the anatomical imagery of CT or MRI, which cannot detect active, viable tumors.

If cancer is found early, it can often be cured. A PET scan can be used in early diagnosis, assisting physicians in determining the best method for treatment. A whole body PET scan may detect whether cancer is isolated to one specific area or has spread to other organs before a treatment path is determined.
PET's are usually done in conjunction with MRI's or CT

No. It is not foolproof.

It is a diagnostic tool that can be used in combination with other diagnostic imaging tools to determine the presence of some types of cancer. It will not detect every area of cancer, especially if the cancer is very small or hidden behind body organs or structures.

My son had three PET scans in a period of 8 months and each one of the scans missed a 4cm mass in the pelvis. It never lit up even though the mass had been present for over 18 months (found when looking back over previous CT).

So, a PET scan can detect some cancer . . but not all cancer all the time.