On my IST a few years back, we had one candidate (From a class of 9) fail the course and have to join the next one, which he promptly failed again and was let go. New trainees I have worked with suggest a similar rate of failure.
With regards to signallers who have passed IST and then been removed from the job in the first year, I know of 2, both of whom had underlying issues which should have been picked up sooner, but for whatever reason, were not.
The main thing is, as others have said, the training program (IST and location specific training) are hard going at times. There is no medal for turning up, the standards to be reached are high for a reason. However, if you apply yourself, work with those around you and ask for help if you are struggling with something, you will succeed.