What could cause very tight tendons under the foot , have tried diag ultrasound, stretching , prp injections for pain there and even shock wave ?

Plantar fascitis: Sound ilke you have a bad case of plantar fascitis- stretching o the feet tendons before you get out of bed in the morning, massage, warm water soaks, injections, ultrasound treatment are all modalities that can be used. Avoid prolonged standing. If all of this does't work, surgery is the last resort- not 100% effective but does work in most cases.

Plantar fasciitis: I assume you are referring to the ligament or plantar fascia. Custom orthotics are the first line of treatment, if they fail then the other treatments in conjunction with orthotics have a good track record. If all else fails then partial plantar fascial release is indicated.

Injury could cause : It, genetics may play a role. Getting an accurate diagnosis is the most crucial thing. Prp won't help stretch a tight tendon or ligament. Stretching is helpful for that, night splints, orthotics etc, you need to determine if you have plantar fascitis, or something else. And get appropriate treatment.

Difficult to know: exactly what you are talking about without examining you. Could be plantar fasciitis if near the heel.