If you are receiving a radar service from a military ATC unit, the controller will limit service:
a. When the ac is close to the lateral or vertical limits of solid radar cover, or within 10 nm of the edge of the radar display.
b. When the ac is close to areas of permanent echoes or weather returns.
c. When the ac is being flown in areas of high traffic density.
d. When a controller considers the performance of the radar is suspect.
e. When a controller is providing a service using SSR data outwith Class A airspace. Provision of RCS in Class A airspace using SSR only is to be specifically authorised in accordance with regulation 601.110.4a
f. Radar control may only be limited in MRSAs.
Note: Primary radar ‘solid cover’ is generally regarded as that portion of the radar's coverage within which a target of small reflecting area (i.e. Hawk) may be expected to paint satisfactorily.
JSP552 235.135.2-3
But he should tell you why, where and for how long, ie
"Radar service limited from the left for 15nm as you pass close to the radar overhead".
or
"Radar service limited from below as you approach the base of radar cover".