A verb for “to try” based on the root √NDEB in notes from the 1960s (PE17/167). As suggested by Christopher Gilson, this appears to be a draft of Tolkien’s attempt to define the phrase “try harder” in Quenya. Of the root √NDEB, Tolkien said it “too obviously = endeavor”, and he wrote another root √RIK below it, with a Sindarin verb form ritha- or raitha. It seems Tolkien here changed √NDEB >> √RIK, but in later notes from 1969 Tolkien had the root √NDAB “endeavor, try, seek opportunity” (PE22/151), indicating that the concept of √NDEB was not entirely abandoned.
Neo-Sindarin: For purposes of Neo-Sindarin, I would mainly use ritha- for “to try (put forth effort)”, but I think dev- is worth retaining with a different connotation: “to try, ✱seek opportunity, experiment”. In other words, you use ritha- “try” when you are uncertain whether you are capable of performing the action, and dev- “try” when you know you are capable, but are uncertain what the result of the action will be.
A verb for “to strive, ✱try” based on the root √RIK in notes from the 1960s (PE17/167). As suggested by Christopher Gilson, this appears to be a draft of Tolkien’s attempt to define the phrase “try harder” in Quenya (ultimately á ricë amrícië). This Sindarin verb has a number of difficult-to-interpret forms. The base verb was given as {reitha >>} raitha, cognate to Q. rihta and thus probably derived from ✶riktā-. However the expected result of primitive ✶riktā- would be S. rítha- or ritha- because of how spirants vocalized and became vowels, so that [ikt] > [ixθ] > [īθ] > [iθ]; compare [N.] critha- “reap” < ᴹ✶k’rikta- (Ety/KIRIK).
Indeed, Tolkien gave several inflicted forms of this verb, all showing ritha-: rithant, rithas, rithessin, rithanen, apparently various transitive and intransitive past forms. Perhaps the uninflected verb form {reitha >>} raitha indicates some kind of a-affection occurring only in the uninflected verb form: [-ikta] > [-ixθa] > [-exθa] > [-eiθa], but if so the expected result would be the deleted {reitha} and not raitha, since [[s|[ei] > [ai] only in final syllables]] in Sindarin.
Neo-Sindarin: Given the rather tenuous nature of this verb, various Neo-Sindarin alternatives have been proposed, including ᴺS. rítha- by Elaran and ᴺS. rig- by myself [Paul Strack] as a direct cognate of Q. ric- “try”. However, I have gradually become more comfortable using S. ritha- as given by Tolkien, since I retain the use of some similarly derived Noldorin verbs such as [N.] critha- “reap” and ᴺS. ritha- “jerk, twitch, snatch” [from N. rhitha-].