It is very common not to be able to get RIS when required from many LARS units because a) they get busy and b) equipment doesn't always work.
For example, climbing through cloud during IMC lesson on Saturday morning between Henley and Woodley when I called and asked for RIS they initially said "yes" but after identifying me on SSR, said that due to equipment problems they had poor primary radar coverage in my area so RIS not possible and downgraded to FIS. Meanwhile as more people called the frequency they started to turn down even FIS for a little while as they were doing the one-armed paper hangar trick.
Its really not their fault - we have to remember that, right or wrong, the LARS service is not funded sufficiently well to give as more than a "best efforts" service. Would that it were otherwise... but the efforts of the guys in the LARS units is appreciated and I would recommend anyone do as many of us have done and pay them a visit to see what they are up against.
Roll on Private IR (there's a rumour!) so that we can get and choose a proper level of service in an airway when / if required. Today we don't have that choice.
Andy