Yes, it is true that whilst a substantial number of ragas can be elaborated / unfolded during alap portion, there are some ragas which require coverage of several notes to project the raga swaroop of the concerned raga. There are several reasons of this as follows:
a. A majority of ragas reveal their swaroop in a few notes and hence a systematic unfolding of the raga in the alap stage can be undertaken on a step by step basis (silsila based badhat). In such ragas, it would be possible to start the alap utilizing सा as the base and unfolding the raga swaroop through a few notes including treatment of a couple of notes below सा and a couple of notes above सा (e.g. raga Puriya, Yaman, Jog etc.)
b. For some ragas like as Jaijaiwanti, Puriya Kalyan, Bihag, Desh, Pilu etc. they cannot be treated in line with ragas mentioned in para 1. It is not possible to fully reveal the swarup of the raga and one is likely to misinterpret the same. For e.g. in Bihag, if the melodic movements are नींसा – नींरेसा – प नी – प सा These phrases could indicate several ragas like Desh, Khamaj, Sarang etc. Therefore it would be necessary to start with phrases like नींसा followed by a meed / murki रेग म ग रे. In this manner, it would be easy to ascertain that the raga is Desh. Thus, in such ragas therefore, it would be essential to have more than one melodic movement to determine the raga.
Conclusion: As would be noted therefore, it is swaroop of the raga which determines whether one can develop it on the basis of silsila or expand the melodic area to cover several phrases.