Implant-Supported Bridge Keeps Falling Off: What is the problem?

3 years ago I had an implant on the lower far left molar #31, the purpose of the implant was to build a bridge that will hold two units. after the oral surgeon completed the implant, he sent me to the dentist and the dentist created a mold and ordered a two unit bridge. Once the bridge was ready he pasted it and stay put for about 4 months and then came off. It has been over three years and at least four times a year the bridge continues to comes off and have to go back to get it back on. It is not only very annoying when it comes off but it is also very painful when it is pasted back. I have been asking the doctor for solutions to the problem and he just says that the problem is not the bridge that it is the implant and the location of the implant. will you please can you give me advise on what may be the problem and if I should see the oral surgeon next time? What are other potential solutions?