Ultrasonic Sensor Measuring Problem

Hi guys
A week ago I tried to make a robot that run away from blocks.Normally an ultrasonic sensor must measure 400 cm.But when i connect ultrasonic sensor to arduino correctly,it measures approximately 150 cm.Then i connected it to my project with two dc motor,it couldnt measure more than 36 cm.It made me crazy.I tried every way to solve problem.I bought new ultrasonic sensor and l293b but I still have the same problem.Please help me what can I do?

Better decoupling?