small_roots doesn't work with larger X?

edit

I already have lower bits of a factor of $N = pq$, and I try to use the following code to factorize $N$

PR.<x> = PolynomialRing(Zmod(n))
nbits = n.nbits()

f = 2^kbits*x + p0
f = f.monic()
roots = f.small_roots(X = 2 ** (nbits // 2 - kbits), beta = 0.3) ### Called it (@)

if roots:
    x0 = roots[0]
    p = gcd(2^kbits*x0 + p0, n)
    print(p)

However it doesn't find any root. But I change (@) to

roots = f.small_roots(X = 2 ** 60, beta = 0.3) ### nbits // 2 - kbits = 165

Then it successfully find the small root of $f$. I read the document but still don't know why the original code doesn't work. Need help QAQ