This is normal,
In case the SDCCH channels are all busy (= high sdcch congestion rate), then the BTS doesn’t have to reply to the MS.
The MS will then retry by itself, by sending a second and then a third CHANNEL REQUEST.
This is a standard mode of operation.
There is feature called “immediate assignment reject” that can be activated, so the BTS will send a message (IMMEDIATE ASSIGNMENT REJECT). This is not mandatory though. It works better this way though (generates less sdcch congestion), so I usually recommend to activate it.